qemu-nbd: Destroy the BlockDriverState properly

Match the bdrv_new() with a bdrv_unref(), just to be tidy.

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

qemu-nbd.c

@@ -773,7 +773,7 @@ int main(int argc, char **argv)
         }
     } while (state != TERMINATED);
 
-    bdrv_close(bs);
+    bdrv_unref(bs);
     if (sockpath) {
         unlink(sockpath);
     }