Check if virtio-gpu provides virgl option, and add qemu command line
formatter.

It is enabled with the existing accel3d attribute:
<model type='virtio' heads='1'>
 <acceleration accel3d='yes'/>
</model>
Signed-off-by: NMarc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>