Description of problem: This is a trivial patch to make comparisons to
None done with 'is'/'is not' instead of '=='/'!=' on the advice in
pep8 (http://www.python.org/dev/peps/pep-0008/):

   - Comparisons to singletons like None should always be done with
     'is' or 'is not', never the equality operators.

Version-Release number of selected component (if applicable):
I made a pathc for git HEAD but I guess it should be applicable to 
the versions of python-virtinst in fedora-14/15.