qemu_validate: Deduplicate code for RNG model check

qemu_validate: Deduplicate code for RNG model check

In my recent commit of 5216304bfe I've moved RNG model check
from domain capabilities validator into qemu validator. During
that I had to basically duplicate RNG 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 RNG models.

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