Support VirtualBox on non-Darwin OSes

Hi, just saw this neat project. Unfortunately it seems like it's set up to only work on OS X right now, but detecting the presence of VirtualBox on Linux seems pretty straightforward so I just added that to vagrant-tramp-ssh. I don't know if this is the right approach (I guess you could potentially have both VMWare and VirtualBox on a Linux host?) but at the least it makes it work for me.