image: Add support for compressed-qcow2 format....* gnu/build/image.scm (convert-disk-image): New procedure.
(genimage): Remove target argument.
* gnu/system/image.scm (system-disk-image): Add support for 'compressed-qcow2
image format. Call "convert-disk-image" to apply image conversions on the
final image. Add "qemu-minimal" to the build inputs.
(system-image): Also add support for 'compressed-qcow2.