Fix windows vbox path

The first commit makes Windows work again (this was the bug that made me back out the vmware refactor in 1.1.1)

the second is to make the Driver responsible for its own parameters.

This will only add the parameters for the currently selected driver - but for now, that's a very expert UX issue we can work on later.
Does this mean that `flags.Parse` is now being called multiple times?  Shouldn't it only be called once?