Update the virtualbox driver to import base boxes into a unique(ish) VM folder to prevent collisions.
use vbox4.2 driver parallel-safe box import in 4.3

uses what @smoyer did for vbox 4.2 in vbox 4.3,
tested against VBoxManage 4.3.2
Merge pull request #2484 from andyfowler/virtualbox-parallel-import

provider/virtualbox: virtualbox parallel box import