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/main.py

@@ -91,7 +91,7 @@ def parse_options():
         help="Create JSON output of the configuration [Default: %default]")
     option("--dot-config", metavar="FILE", default="config.dot",
         help="Create DOT & pdf outputs of the configuration [Default: %default]")
-    option("--dot-dvfs-config", metavar="FILE", default="config_dvfs.dot",
+    option("--dot-dvfs-config", metavar="FILE", default=None,
         help="Create DOT & pdf outputs of the DVFS configuration" + \
              " [Default: %default]")