qemu-img: Let info print block graph

For every node in the backing chain, collect its BlockGraphInfo struct
using bdrv_query_block_graph_info().  Print all nodes' information,
indenting child nodes and labelling them with a path constructed from
the child names leading to the node from the root (e.g. /file/file).

Note that we open each image with BDRV_O_NO_BACKING, so its backing
child is omitted from this graph, and thus presented in the previous
manner: By simply concatenating all images' information, separated with
blank lines.

This affects two iotests:
- 065: Here we try to get the format node's format specific information.
  The pre-patch code does so by taking all lines from "Format specific
  information:" until an empty line.  This format specific information
  is no longer followed by an empty line, though, but by child node
  information, so limit the range by "Child node '/file':".
- 302: Calls qemu_img() for qemu-img info directly, which does not
  filter the output, so the child node information ends up in the
  output.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220620162704.80987-12-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/065

@@ -56,7 +56,7 @@ class TestQemuImgInfo(TestImageInfoSpecific):
     def test_human(self):
         data = qemu_img('info', '--output=human', test_img).stdout.split('\n')
         data = data[(data.index('Format specific information:') + 1)
-                    :data.index('')]
+                    :data.index("Child node '/file':")]
         for field in data:
             self.assertTrue(re.match('^ {4}[^ ]', field) is not None)
         data = [line.strip() for line in data]

tests/qemu-iotests/302.out

@@ -4,6 +4,11 @@ image: nbd+unix:///exp?socket=SOCK_DIR/PID-nbd-sock
 file format: raw
 virtual size: 448 KiB (458752 bytes)
 disk size: unavailable
+Child node '/file':
+    image: nbd+unix:///exp?socket=SOCK_DIR/PID-nbd-sock
+    file format: nbd
+    virtual size: 448 KiB (458752 bytes)
+    disk size: unavailable
 
 === Converted image info ===
 image: TEST_IMG