conf: add support for 'blob' in virtio video device

conf: add support for 'blob' in virtio video device

Add the ability to enable blob resources for the virtio video device.
This will accelerate the display path due to less or no copying of pixel
data.

Blob resource support can be enabled with e.g.:

    <video>
      <model type='virtio' blob='on'/>
    </video>

Some additional background information about blob resources:
https://lists.freedesktop.org/archives/dri-devel/2020-August/275972.html
https://www.kraxel.org/blog/2021/05/virtio-gpu-qemu-graphics-update/

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2032406

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>