virtio-blk: Handle immediate flush failure properly

Fix virtio-blk to use the usual completion path that involves werror handling
instead of directly completing the request in cases where bdrv_aio_flush
returns NULL.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/virtio-blk.c

@@ -273,7 +273,7 @@ static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)
 
     acb = bdrv_aio_flush(req->dev->bs, virtio_blk_flush_complete, req);
     if (!acb) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
+        virtio_blk_flush_complete(req, -EIO);
     }
 }