virtio-gpu: Don't require udmabuf when blobs and virgl are enabled

The udmabuf usage is mandatory when virgl is disabled and blobs feature
enabled in the Qemu machine configuration. If virgl and blobs are enabled,
then udmabuf requirement is optional. Since udmabuf isn't widely supported
by a popular Linux distros today, let's relax the udmabuf requirement for
blobs=on,virgl=on. Now, a full-featured virtio-gpu acceleration is
available to Qemu users without a need to have udmabuf available in the
system.

Reviewed-by: Antonio Caggiano <antonio.caggiano@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>