qemu_validate: Deduplicate code for video model check

qemu_validate: Deduplicate code for video model check

In my recent commit of a33279daa8 I've moved video model check
from domain capabilities validator into qemu validator. During
that I had to basically duplicate video model to qemuCaps checks.
Problem with this approach is that after my commit qemu validator
and domCaps are disconnected and thus domCaps might report (in
general) different set of supported video models.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>