virtio-ccw: Add missing blk chs properties.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
s390-virtio, virtio-ccw: Add config_wce for virtio-blk.

There's no reason why we wouldn't want to make the cache mode
configurable.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>