virtio/s390: implement PM operations for virtio_ccw

Suspend/Resume to/from disk currently fails. Let us wire
up the necessary callbacks. This is mostly just forwarding
the requests to the virtio drivers. The only thing that
has to be done in virtio_ccw itself is to re-set the
virtio revision.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20171207141102.70190-2-borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
[CH: merged <20171218083706.223836-1-borntraeger@de.ibm.com> to fix
!CONFIG_PM configs]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>