virtio: check for vring setup in virtio_queue_empty

If the vring has not been set up, there is nothing in the virtqueue.
virtio_queue_host_notifier_aio_poll calls virtio_queue_empty even in
this case; we have to filter it out just like virtio_queue_notify_aio_vq.
Reported-by: NGerd Hoffmann <kraxel@redhat.com>
Tested-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Tested-by: NLaszlo Ersek <lersek@redhat.com>
Tested-by: NCornelia Huck <cornelia.huck@de.ibm.com>

hw/virtio/virtio.c

@@ -2291,7 +2291,7 @@ static bool virtio_queue_host_notifier_aio_poll(void *opaque)
     VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
     bool progress;
 
-    if (virtio_queue_empty(vq)) {
+    if (!vq->vring.desc || virtio_queue_empty(vq)) {
         return false;
     }