qom: Plug memory leak in "info qom-tree"

qom: Plug memory leak in "info qom-tree"

Commit e8c9e65816 "qom: Make "info qom-tree" show children sorted"
created a memory leak, because I didn't realize
object_get_canonical_path_component()'s value needs to be freed.

Reproducer:

    $ qemu-system-x86_64 -nodefaults -display none -S -monitor stdio
    QEMU 5.0.50 monitor - type 'help' for more information
    (qemu) info qom-tree

This leaks some 4500 path components, 12-13 characters on average,
i.e. roughly 100kBytes depending on the allocator.  A couple of
hundred "info qom-tree" here, a couple of hundred there, and soon
enough we're talking about real memory.

Plug the leak.

Fixes: e8c9e65816f5dbfe18ad3b2be938d0d8192d459a
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reported-by: Reviewed-by: Li Qiang <liq3ea@gmail.com> [sent same patch]
Message-Id: <20200714160202.3121879-3-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>