[xenserver] updated create_sr request documentation, fixed default values
[xenserver|tests] Added create_sr request tests
[xenserver|tests] Added destroy_sr request tests
[xenserver] add missing PBD 'currently_attached' attribute
[xenserver|tests] add unplug_pbd request tests
[xenserver|tests] tests PBDs plug/unplug operation
[xenserver|tests] added more pool tests to cover the new functionality
[xenserver|tests] added missing SotorageRepository tests