Compatibility with Parallels provider

This commit adds compatibility with [Parallels provider](https://github.com/yshahin/vagrant-parallels)
I've checked it by myself and all existing features is working fine for this environment:
- Vagrant 1.4.0
- 'vagrant-parallels' v0.1.0
- Parallels Desktop 8 and 9
- Host: OS X 10.9
- Guest: Windows Server 2012 R2

P.s. I'm ready to support this compatibility in future.