misc: Bail out of DVFS dot if we cannot resolve the domains

This changeset updates the dot output to bail out if it is unable to
resolve the voltage or clock domains (which will cause it to raise an
AttributeError). Additionally, the DVFS dot output is disabled by
default for speed purposes.

Minor fixup for 0aeca8f.

src/python/m5/simulate.py

@@ -129,7 +129,8 @@ def instantiate(ckpt_dir=None):
     # We want to generate the DVFS diagram for the system. This can only be
     # done once all of the CPP objects have been created and initialised so
     # that we are able to figure out which object belongs to which domain.
-    do_dvfs_dot(root, options.outdir, options.dot_dvfs_config)
+    if options.dot_dvfs_config:
+        do_dvfs_dot(root, options.outdir, options.dot_dvfs_config)
 
     # We're done registering statistics.  Enable the stats package now.
     stats.enable()