virtio/s390: enable packed ring

Nothing precludes to accepting  VIRTIO_F_RING_PACKED any more.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>