virtio: make virtio_delete_queue idempotent

Let's make sure calling this twice is harmless -
no known instances, but seems safer.

Suggested-by: Pan Nengyuan <pannengyuan@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -2337,6 +2337,7 @@ void virtio_delete_queue(VirtQueue *vq)
     vq->handle_output = NULL;
     vq->handle_aio_output = NULL;
     g_free(vq->used_elems);
+    vq->used_elems = NULL;
 }
 
 void virtio_del_queue(VirtIODevice *vdev, int n)