[xenserver] added create_sr request (create Storage Repository)
[xenserver] added destroy_sr request (destroy Storage Repository)
[xenserver] added unplug_pbd request
[xenserver] added new requests to compute service

- unplug_pbd
- destroy_sr
- create_sr
[xenserver] added unplug method to PBD model
[xenserver] missing Pool model attribute, new methods

- Added missing suspend_image_sr attribute
- added default_sr getter/setter
- added suspend_image_sr getter/setter
- added generic set_attribute method
[xenserver] added new methods to StorageRepository model

- destroy/save operations
- generic set_attribute method
[xenserver|docs] Added new example that covers a Citrix KB ctx116324 article

Translated http://support.citrix.com/article/ctx116324 to fog/xenserver
[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
Merge pull request #1432 from rubiojr/new-xenserver-features

New xenserver features and examples