virtio-ccw: using VIRTIO_NO_VECTOR instead of 0 for invalid virtqueue

It's a bad idea to need to use vector 0 for invalid virtqueue. So this patch
changes to using VIRTIO_NO_VECTOR instead.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
CC: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Acked-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/s390x/virtio-ccw.c

@@ -281,7 +281,7 @@ static int virtio_ccw_set_vqs(SubchDev *sch, uint64_t addr, uint32_t align,
 
     virtio_queue_set_addr(vdev, index, addr);
     if (!addr) {
-        virtio_queue_set_vector(vdev, index, 0);
+        virtio_queue_set_vector(vdev, index, VIRTIO_NO_VECTOR);
     } else {
         /* Fail if we don't have a big enough queue. */
         /* TODO: Add interface to handle vring.num changing */