[xenserver] StorageRepository.save should use sane defaults

- Added required tests
- Minor StorageRepository tests refactoring
[xenserver] add missing host operations (enable/disable, reboot, shutdown)
Merge pull request #1449 from rubiojr/missing-host-operations

[xenserver] add missing host operations (enable/disable, reboot, shutdown)
Merge pull request #1448 from rubiojr/xenserver-sr-fixes

[xenserver] StorageRepository.save should use sane defaults