conf: domain: move video type validation to DeviceDefValidate
conf: domain: move video type validation to DeviceDefValidate

This allows drivers to set their own default. But if a driver neglects
to fill one in, we still error like we previously would at parse time.

Signed-off-by: Cole Robinson <7cd45ff6ad1fb7cf556a02797d774247851499f8@redhat.com>

fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,fabianfreyer/libvirt,libvirt/libvirt,zippy2/libvirt,andreabolognani/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jfehlig/libvirt,jardasgit/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,eskultety/libvirt,crobinso/libvirt,crobinso/libvirt,crobinso/libvirt,eskultety/libvirt,crobinso/libvirt,jfehlig/libvirt,eskultety/libvirt,fabianfreyer/libvirt,eskultety/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,libvirt/libvirt,jardasgit/libvirt,zippy2/libvirt,nertpinx/libvirt,jfehlig/libvirt,zippy2/libvirt,andreabolognani/libvirt,olafhering/libvirt,nertpinx/libvirt,zippy2/libvirt,jfehlig/libvirt,jardasgit/libvirt,eskultety/libvirt,olafhering/libvirt