Reintroduce VirtualBox support

virtualbox-guest-utils was removed in 873f64e, and the guest modules building in 506fe82, as VirtualBox was removed from Jessie, which broke the build.

Feature Branch: feature/jessie

Original created by @intrigeri on 7602 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information