s390x/virtio-ccw: support virtio-1 set_vq format

Support the new CCW_CMD_SET_VQ format for virtio-1 devices.

While we're at it, refactor the code a bit and enforce big endian
fields (which had always been required, even for legacy).

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