memdev: remove "id" property

The "id" property is unnecessary and can be replaced simply with
object_get_canonical_path_component.  This patch mostly undoes commit
e1ff3c67e8 ("monitor: fix qmp/hmp query-memdev not reporting IDs of
memory backends", 2017-01-12).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/numa.c b/numa.c
index 70b150e..aac22a9 100644
--- a/numa.c
+++ b/numa.c
@@ -580,7 +580,7 @@
 
         m->value = g_malloc0(sizeof(*m->value));
 
-        m->value->id = object_property_get_str(obj, "id", NULL);
+        m->value->id = object_get_canonical_path_component(obj);
         m->value->has_id = !!m->value->id;
 
         m->value->size = object_property_get_uint(obj, "size",