Add XenApi for Xen/XenServer

Add support for XenAPI via xenapi-ruby gem.
Initial methods scaffolded from VMWare vSphere and Fog

Tested against XenServer6.1 and 6.0.2

Testing procedure:
```
controller = ::Lab::Controllers::VmController.new
controller.build_from_running(opts)
controller[<name from name_label field of a running VM>]
```

TODO: OpenSSL still refuses to accept self-signed certificates.
Would also be nice to rewire XMLRPC::Client to run over Rex::Socket
This may also help to maintain session keepalives and query many
endpoints conrurrently for faster processing.