events: Make trace events happen at the right priority.

Also, while we're at it, remember that priorities are in the Event class
and add a disable method to disable tracing.

src/python/m5/trace.py

@@ -32,6 +32,9 @@ import util
 
 from internal.trace import clear, output, set, ignore
 
+def disable():
+    internal.trace.cvar.enabled = False
+
 def enable():
     internal.trace.cvar.enabled = True