qom: Fix ambiguous path detection when ambiguous=NULL

object_resolve_path*() ambiguous path detection breaks when
ambiguous==NULL and the object tree have 3 objects of the same type and
only 2 of them are under the same parent.  e.g.:

 /container/obj1 (TYPE_FOO)
 /container/obj2 (TYPE_FOO)
 /obj2 (TYPE_FOO)

With the above tree, object_resolve_path_type("", TYPE_FOO, NULL) will
incorrectly return /obj2, because the search inside "/container" will
return NULL, and the match at "/obj2" won't be detected as ambiguous.

Fix that by always calling object_resolve_partial_path() with a non-NULL
ambiguous parameter.

Test case included.

Reported-by: Igor Mammedov <imammedo@redhat.com>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170707213052.13087-3-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>