qapi: Add "backing" to BlockStats

Currently there is no way to query BlockStats of the backing chain. This
adds "backing" field into BlockStats to make it possible.

The comment of "parent" is reworded.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qapi.c

@@ -318,6 +318,11 @@ BlockStats *bdrv_query_stats(const BlockDriverState *bs)
         s->parent = bdrv_query_stats(bs->file);
     }
 
+    if (bs->backing_hd) {
+        s->has_backing = true;
+        s->backing = bdrv_query_stats(bs->backing_hd);
+    }
+
     return s;
 }
 

qapi-schema.json

@@ -1022,15 +1022,17 @@
 #
 # @stats:  A @BlockDeviceStats for the device.
 #
-# @parent: #optional This may point to the backing block device if this is a
-#          a virtual block device.  If it's a backing block, this will point
-#          to the backing file is one is present.
+# @parent: #optional This describes the file block device if it has one.
+#
+# @backing: #optional This describes the backing block device if it has one.
+#           (Since 2.0)
 #
 # Since: 0.14.0
 ##
 { 'type': 'BlockStats',
   'data': {'*device': 'str', 'stats': 'BlockDeviceStats',
-           '*parent': 'BlockStats'} }
+           '*parent': 'BlockStats',
+           '*backing': 'BlockStats'} }
 
 ##
 # @query-blockstats: