Default to tracing being disabled in C++, it will be turned

on in python.  Fix the trace start code so it actually starts
when it is suppsed to.  Make the Exec tracing stuff obey the
trace enabled flag.

--HG--
extra : convert_revision : 634ba0b4f52345d4bf40d43e239cef7ef43e7691

src/cpu/exetrace.hh

@@ -157,6 +157,9 @@ getInstRecord(Tick when, ThreadContext *tc, const StaticInstPtr staticInst,
     if (!IsOn(ExecEnable))
         return NULL;
 
+    if (!Trace::enabled)
+        return NULL;
+
     if (!IsOn(ExecSpeculative) && tc->misspeculating())
         return NULL;