KVM: s390: enable virtio-ccw revision 1

Now that virtio-ccw has everything needed to support virtio 1.0 in
place, try to enable it if the host supports it.

Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>