sim: Give the NullSimObject singleton a _name.

The name is 'Null', and is what __str__ returns.

Change-Id: I113f52496f5e9133b8d03206289b719fda003582
Reviewed-on: https://gem5-review.googlesource.com/4845
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/python/m5/params.py

@@ -1694,6 +1694,7 @@ class MemoryBandwidth(float,ParamValue):
 # only one copy of a particular node
 class NullSimObject(object):
     __metaclass__ = Singleton
+    _name = 'Null'
 
     def __call__(cls):
         return cls
@@ -1721,7 +1722,7 @@ class NullSimObject(object):
         yield None
 
     def __str__(self):
-        return 'Null'
+        return self._name
 
     def config_value(self):
         return None