mirror: Fix inconsistent backing AioContext for after mirroring

mirror_complete opens the backing chain, which should have the same
AioContext as the top when using iothreads. Make the code guarantee
this, which fixes a failed assertion in bdrv_attach_child.
Signed-off-by: Nsochin.jiang <sochin.jiang@huawei.com>
Message-id: 1498475064-39816-1-git-send-email-sochin.jiang@huawei.com
[mreitz: Reworded commit message]
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block.c

@@ -2185,6 +2185,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
         ret = -EINVAL;
         goto free_exit;
     }
+    bdrv_set_aio_context(backing_hd, bdrv_get_aio_context(bs));
 
     /* Hook up the backing file link; drop our reference, bs owns the
      * backing_hd reference now */