FEAT: proof of concept to mound guest on windows.

This is just a proof of concept on mounting vbox additions (as veewee used to do). I think it is easier to deal with on windows.

The code is not ready to be merged, but since I don't know any go, I figure it would be easier for packer developers to do this correctly. That hack allowed me to build a vagrant windows box that I could use in vagrant 1.2.3 (+ vagrant-windows from https://github.com/WinRb/vagrant-windows)