vhost: fix invalid downcast

virtio_queue_get_desc_addr returns 64-bit hwaddr while int is usually 32-bit.
If returned hwaddr is not equal to 0 but least-significant 32 bits are
equal to 0 then this code will not actually stop running queue.
Signed-off-by: NYury Kotov <yury-kotov@yandex-team.ru>
Acked-by: NJia He <hejianet@gmail.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit fa4ae4be)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/virtio/vhost.c

@@ -1073,10 +1073,8 @@ static void vhost_virtqueue_stop(struct vhost_dev *dev,
         .index = vhost_vq_index,
     };
     int r;
-    int a;
 
-    a = virtio_queue_get_desc_addr(vdev, idx);
-    if (a == 0) {
+    if (virtio_queue_get_desc_addr(vdev, idx) == 0) {
         /* Don't stop the virtqueue which might have not been started */
         return;
     }