Enhancements for Parallels builder

I've done the following changes:
* Deprecated config parameter `guest_os_type`, because it is not required if 'guest_os_distribution' is specified. It didn't remove it from the `config` struct to save a backward compatibility, so that there will not be an error if `guest_os_type` is specified in template.json.
* Added platform check. Parallels builders are working only on `darwin` platform
* Added compatibility with Parallels Desktop 10. I've made this changeset early on previous week, before than #1438 has been created. So I think these changes are more general and new `Parallels10Driver` struct will be useful in the future.

Any suggestions are welcome!

cc:/ @rickard-von-essen