Qemu supports *xen* video device only with XEN and this code was part
of xenner code.  We dropped support for xenner in commit de9be0ab.

Before this patch if you used 'xen' video type you ended up with
domain without any video device at all.  Now we don't allow to start
such domain.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>