Add temporary fix for VirtualBox Additions 4.3.10 issue

VirtualBox Guest Additions version 4.3.10 misses to add a symlink during
installation which results in Vagrant not being able to mount shared
folders. See VirtualBox ticket #12879:
https://www.virtualbox.org/ticket/12879.

Signed-off-by: Gregor Zurowski <gregor@zurowski.org>