config: Fix handling of parents for simobject vectors

SimObjectVector objects did not provide the same interface to
the _parent attribute through get_parent() like a normal
SimObject.  It also handled assigning a _parent incorrectly
if objects in a SimObjectVector were changed post-creation,
leading to errors later when the simulator tried to execute.
This patch fixes these two omissions.

src/python/m5/SimObject.py

@@ -784,6 +784,11 @@ class SimObject(object):
         self._parent = parent
         self._name = name
 
+    # Return parent object of this SimObject, not implemented by SimObjectVector
+    # because the elements in a SimObjectVector may not share the same parent
+    def get_parent(self):
+        return self._parent
+
     # Also implemented by SimObjectVector
     def get_name(self):
         return self._name