Allow a QEMU host to set the time and shutdown Guix guests.  Styled
after the patch from the Nix package:

https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/virtualization/qemu/fix-qemu-ga.patch

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -84,6 +84,7 @@ static void ga_wait_child(pid_t pid, int *status, Error **errp)
 void qmp_guest_shutdown(bool has_mode, const char *mode, Error **errp)
 {
     const char *shutdown_flag;
+    const char *command;
     Error *local_err = NULL;
     pid_t pid;
     int status;
@@ -101,10 +102,13 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, Error **errp)
     slog("guest-shutdown called, mode: %s", mode);
     if (!has_mode || strcmp(mode, "powerdown") == 0) {
         shutdown_flag = powerdown_flag;
+        command = "shutdown";
     } else if (strcmp(mode, "halt") == 0) {
         shutdown_flag = halt_flag;
+        command = "halt";
     } else if (strcmp(mode, "reboot") == 0) {
         shutdown_flag = reboot_flag;
+        command = "reboot";
     } else {
         error_setg(errp,
                    "mode is invalid (valid values are: halt|powerdown|reboot");
@@ -123,6 +127,11 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, Error **errp)
         execl("/sbin/shutdown", "shutdown", shutdown_flag, "-g0", "-y",
               "hypervisor initiated shutdown", (char *)NULL);
 #else
+        /* try Guix’s shutdown/halt/reboot first */
+        char *path = g_strdup_printf("/run/current-system/profile/sbin/%s", command);
+        execl(path, command, (char *)NULL);
+        g_free(path);
+
         execl("/sbin/shutdown", "shutdown", "-h", shutdown_flag, "+0",
                "hypervisor initiated shutdown", (char *)NULL);
 #endif
@@ -159,10 +168,12 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
     Error *local_err = NULL;
     struct timeval tv;
     static const char hwclock_path[] = "/sbin/hwclock";
+    static const char hwclock_path_guix[] = "/run/current-system/profile/sbin/hwclock";
     static int hwclock_available = -1;
 
     if (hwclock_available < 0) {
-        hwclock_available = (access(hwclock_path, X_OK) == 0);
+        hwclock_available = (access(hwclock_path_guix, X_OK) == 0) ||
+                            (access(hwclock_path, X_OK) == 0);
     }
 
     if (!hwclock_available) {
@@ -208,6 +219,8 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
 
         /* Use '/sbin/hwclock -w' to set RTC from the system time,
          * or '/sbin/hwclock -s' to set the system time from RTC. */
+        execl(hwclock_path_guix, "hwclock", has_time ? "-w" : "-s",
+               NULL);
         execl(hwclock_path, "hwclock", has_time ? "-w" : "-s", NULL);
         _exit(EXIT_FAILURE);
     } else if (pid < 0) {
ty): Likewise.
(machine-check-building-for-appropriate-system): Likewise.
(deploy-managed-host): Likewise.
(maybe-raise-unsupported-configuration-error): Likewise.
* gnu/packages.scm (search-patch): Likewise.
* gnu/services.scm (%service-with-default-value): Likewise.
(files-&gt;etc-directory): Likewise.
(fold-services): Likewise.
* gnu/system.scm (locale-name-&gt;definition*): Likewise.
* gnu/system/mapped-devices.scm (check-device-initrd-modules): Likewise.
(check-luks-device): Likewise.
* guix/channels.scm (latest-channel-instance): Likewise.
* guix/cve.scm (json-&gt;cve-items): Likewise.
* guix/git-authenticate.scm (commit-signing-key): Likewise.
(commit-authorized-keys): Likewise.
(authenticate-commit): Likewise.
(verify-introductory-commit): Likewise.
* guix/remote.scm (remote-pipe-for-gexp): Likewise.
* guix/scripts/graph.scm (assert-package): Likewise.
* guix/scripts/offload.scm (private-key-from-file*): Likewise.
* guix/ssh.scm (authenticate-server*): Likewise.
(open-ssh-session): Likewise.
(remote-inferior): Likewise.
* guix/ui.scm (matching-generations): Likewise.
* guix/upstream.scm (package-update): Likewise.
* tests/channels.scm ("latest-channel-instances, missing introduction for 'guix'"):
Catch 'formatted-message?'.
("authenticate-channel, wrong first commit signer"): Likewise.
* tests/lint.scm ("patches: not found"): Adjust message string.
* tests/packages.scm ("patch not found yields a run-time error"): Catch
'formatted-message?'.
* guix/lint.scm (check-patch-file-names): Handle 'formatted-message?'.
(check-derivation): Ditto.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-07-25 19:11:36 +0200'>2020-07-25</span></td><td><a href='/guix/commit/gnu.scm?id=f9a8dd053c4e0fd1fc4b64291bb90de36520b3bc'>utils: Move '&amp;fix-hint' to (guix diagnostics).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/utils.scm (&amp;fix-hint): Move to...
* guix/diagnostics.scm (&amp;fix-hint): ... here.
* gnu.scm: Adjust imports accordingly.
* gnu/system/mapped-devices.scm: Likewise.
* guix/channels.scm: Likewise.
* guix/profiles.scm: Likewise.
* guix/scripts/system/reconfigure.scm: Likewise.
* guix/ssh.scm: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-07-25 19:11:36 +0200'>2020-07-25</span></td><td><a href='/guix/commit/gnu.scm?id=a5e2fc73760a2ae023f2e56bdbf8025971f90e64'>utils: Move &lt;location&gt; and '&amp;error-location' to (guix diagnostics).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/utils.scm (&lt;location&gt;, source-properties-&gt;location)
(location-&gt;source-properties, &amp;error-location): Move to...
* guix/diagnostics.scm: ... here.
* gnu.scm: Adjust imports accordingly.
* gnu/machine.scm: Likewise.
* gnu/system.scm: Likewise.
* gnu/tests.scm: Likewise.
* guix/inferior.scm: Likewise.
* tests/channels.scm: Likewise.
* tests/packages.scm: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>