virtio: check for vring setup in virtio_queue_empty

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: Gerd Hoffmann <[email protected]>
Tested-by: Alex Williamson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Tested-by: Laszlo Ersek <[email protected]>
Tested-by: Cornelia Huck <[email protected]>