stats: track if the stats have been enabled and prevent requesting master id

Track the point in the initialization where statistics have been registered.
After this point registering new masterIds can no longer work as some
SimObjects may have sized stats vectors based on the previous value. If someone
tries to register a masterId after this point the simulator executes fatal().

src/python/m5/stats/__init__.py

@@ -82,6 +82,8 @@ def enable():
         stats_dict[stat.name] = stat
         stat.enable()
 
+    internal.stats.enable();
+
 def prepare():
     '''Prepare all stats for data access.  This must be done before
     dumping and serialization.'''

src/python/swig/stats.i

@@ -146,6 +146,8 @@ void schedStatEvent(bool dump, bool reset,
                     Tick when = curTick(), Tick repeat = 0);
 
 void processResetQueue();
+void enable();
+bool enabled();
 
 std::list<Info *> &statsList();