- added new tests
* [xenserver|compute] VBD fixes and cleanup
- VBD.metrics now returns nil when VBD not attached
- VBD tests fixes and cleanup
* [xenserver|compute] cleanup tests and add new helpers
* Added missing exceptions NotFound and RequestFailed
* connection.request now raises exception if request failed
* refactored most get_* request into get_record and get_records
* Compute.default_template more robust
* Base parser now replaces OpaqueRef:NULL with nil
* create_server request fixes