bails out of virtualbox driver quickly if it doesn't exist.

Signed-off-by: Simon Thulbourn <simon+github@thulbourn.com>
bails out of virtualbox driver if VBoxManage doesn't exist

Signed-off-by: Simon Thulbourn <simon+github@thulbourn.com>
Merge pull request #40 from sthulb/vbox-driver-quick-failure

VirtualBox bailout