"info qom-tree" prints children in unstable order.  This is a pain
when diffing output for different versions to find change.  Print it
sorted.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200527084754.7531-3-armbru@redhat.com>
Tested-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>