Only check for for relative path on virtio devices.  Otherwise it could
break driver loading in some circumstances, notably the IceCat sandbox.

https://gitlab.freedesktop.org/mesa/drm/-/issues/39

Taken from upstream:
https://gitlab.freedesktop.org/mesa/drm/-/commit/57df07572ce45a1b60bae6fb89770388d3abd6dd

diff --git a/xf86drm.c b/xf86drm.c
--- a/xf86drm.c
+++ b/xf86drm.c
@@ -3103,15 +3103,18 @@ static int drmParseSubsystemType(int maj, int min)
     int subsystem_type;
 
     snprintf(path, sizeof(path), "/sys/dev/char/%d:%d/device", maj, min);
-    if (!realpath(path, real_path))
-        return -errno;
-    snprintf(path, sizeof(path), "%s", real_path);
 
     subsystem_type = get_subsystem_type(path);
+    /* Try to get the parent (underlying) device type */
     if (subsystem_type == DRM_BUS_VIRTIO) {
+        /* Assume virtio-pci on error */
+        if (!realpath(path, real_path))
+            return DRM_BUS_VIRTIO;
         strncat(path, "/..", PATH_MAX);
         subsystem_type = get_subsystem_type(path);
-    }
+        if (subsystem_type < 0)
+            return DRM_BUS_VIRTIO;
+     }
     return subsystem_type;
 #elif defined(__OpenBSD__) || defined(__DragonFly__) || defined(__FreeBSD__)
     return DRM_BUS_PCI;
@@ -3920,6 +3923,7 @@ process_device(drmDevicePtr *device, const char *d_name,
 
     switch (subsystem_type) {
     case DRM_BUS_PCI:
+    case DRM_BUS_VIRTIO:
         return drmProcessPciDevice(device, node, node_type, maj, min,
                                    fetch_deviceinfo, flags);
     case DRM_BUS_USB:
 action='/guix/log/tests/cve-sample.json'>
<input type='hidden' name='id' value='c4eccd20664a0f9d9134d98a5b430f67e84176c7'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=c4eccd20664a0f9d9134d98a5b430f67e84176c7'>root</a>/<a href='/guix/log/tests?id=c4eccd20664a0f9d9134d98a5b430f67e84176c7'>tests</a>/<a href='/guix/log/tests/cve-sample.json?id=c4eccd20664a0f9d9134d98a5b430f67e84176c7'>cve-sample.json</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/cve-sample.json?id=c4eccd20664a0f9d9134d98a5b430f67e84176c7&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-21 17:47:35 +0100'>2020-12-21</span></td><td><a href='/guix/commit/tests/cve-sample.json?id=9608f4003dedd8dfe99327c15668ca1a43ebd93b'>tests: Fix malformed JSON.</a><span class='msg-avail'>...<span class='msg-tooltip'>Guile-JSON 4.3.2 would parse in spite of these typos, but 4.4.1 is
stricter.

* tests/swh.scm (%directory-entries): Add missing comma.
* tests/cve-sample.json: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>