Update Vagrantfile

VirtualBox 4.3.10 has a known bug;  save people from needing to find this, or (worse) forgetting the "sudo" in the "ln -s..." command.   The comment refers to VBox bug report, so it should be simple to remove this 6-12 months down the road.

(This only applies to this Vagrantfile I believe;  does not seem to be pertinent to base/devstack/Vagrantfile, which has no shell provisioning, nor either of the */fullstack Vagrantfiles)