Full XML-based instance checkpoint, serial for vols

- Moved away from instance.checkpoint for instance.xml
- When instance.checkpoint is present, upgrade takes place
- Expanded unit test in xml.c to exercise whole instance struct
- Minor Makefile changes anticipating build-time unit tests
- Minor log-level tweaks (now that instance.xml is updated often)
- Added 'serial' number to volumes

Fixes EUCA-6901, EUCA-7610, EUCA-4947