virtio/s390: support READ_STATUS command for virtio-ccw

As virtio-1 introduced the possibility of the device manipulating the
status byte, revision 2 of the virtio-ccw transport introduced a means
of getting the status byte from the device via READ_STATUS. Let's wire
it up for revisions >= 2 and fall back to returning the stored status
byte if not supported.

Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>