block/qapi: Introduce BlockGraphInfo

Introduce a new QAPI type BlockGraphInfo and an associated
bdrv_query_block_graph_info() function that recursively gathers
BlockNodeInfo objects through a block graph.

A follow-up patch is going to make "qemu-img info" use this to print
information about all nodes that are (usually implicitly) opened for a
given image file.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220620162704.80987-8-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qapi/block-core.json

@@ -307,6 +307,41 @@
       '*backing-image': 'ImageInfo'
   } }
 
+##
+# @BlockChildInfo:
+#
+# Information about all nodes in the block graph starting at some node,
+# annotated with information about that node in relation to its parent.
+#
+# @name: Child name of the root node in the BlockGraphInfo struct, in its role
+#        as the child of some undescribed parent node
+#
+# @info: Block graph information starting at this node
+#
+# Since: 8.0
+##
+{ 'struct': 'BlockChildInfo',
+  'data': {
+      'name': 'str',
+      'info': 'BlockGraphInfo'
+  } }
+
+##
+# @BlockGraphInfo:
+#
+# Information about all nodes in a block (sub)graph in the form of BlockNodeInfo
+# data.
+# The base BlockNodeInfo struct contains the information for the (sub)graph's
+# root node.
+#
+# @children: Array of links to this node's child nodes' information
+#
+# Since: 8.0
+##
+{ 'struct': 'BlockGraphInfo',
+  'base': 'BlockNodeInfo',
+  'data': { 'children': ['BlockChildInfo'] } }
+
 ##
 # @ImageCheck:
 #