[libvirt] [test-API PATCH 0/3] make clean function optional

[Guannan Ren]

The set of patches is to make the writing of clean function optional.
If a testcase which doesn't dirty the testing environment, then the 
TESTCASE_clean function could be optional or omitted.

Because loggers in python are static objects managed by the module itself.
When you create one, it won't be removed until the shell quit. In test-API
there are two loggers which are configured with two handlers for each.
we need to find a way to destruct these two loggers in order no to affect
other application.  The idea is to clear the handlers of each logger in
destructor of CaseLog and EnvLog.

    def __del__(self):
        self.logger.handlers = []


rename some unclear function and variables.
for example: repos/domain/start.py

domain -> mod
start.py -> case
start() -> func