The 'virtio' argument was misleadingly implying that it's true for all
virtio devices, but that's not the case. 'virtio-vga(-gl)' is a virtio
device but doesn't accept the usual bus-dependant suffix.
Add a comment for 'qemuDeviceVideoGetModel' and another boolean
'virtioBusSuffix' which carries the above meaning so that the 'virtio'
argument can be fixed (it will be used later).
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>