Add unit tests for guestagent.volume.py

- more clean up on test_manager.py
- Migrate guestagent tests to unittest folder
- modify guestagent.manager.py so it recognize '_'

Bug # 1090139

Change-Id: I718a5212cd07c8bdb4e71796ba384574ce8275e9
Merge "Add unit tests for guestagent.volume.py"