WinRM communicator uses wrong host and port

- Plugin will attempt to connect to WinRM host `127.0.0.1` rather than the IP gotten from XenServer tools.
- Plugin incorrectly sets the default port, and if the `winrm_port` is given in the config, it is ignored.

I've made a hackish patch for this @ https://github.com/dongyuzheng/packer-builder-xenserver/tree/winrm_fix
