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>