qemu: use domain caps to validate video device model

qemu: use domain caps to validate video device model

As suggested by Cole, this patch uses the domain capabilities to
validate the supported video model types. This allows us to remove the
model type validation from qemu_process.c and qemu_domain.c and
consolidates it all in a single place that will automatically adjust
when new domain capabilities are added.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>