Change revert to revert_to

I noticed that in server.rb(1), line 168, you used revert_to_vm( alias_method to revert_to_server), but the request in xen_server.rb(2) was revert_server. Change it, because you said that revert will be deprecate.


https://github.com/fog/fog-xenserver/blob/master/lib/fog/compute/xen_server/models/server.rb#L168
https://github.com/fog/fog-xenserver/blob/master/lib/fog/compute/xen_server.rb#L283