Check what to do with VirtualBox on Stretch

It was removed from testing due to https://bugs.debian.org/794466, so we now install it from sid (acb91b93f1fee1c0865a74007efcfaa76c9f462b).

Feature Branch: wip/bugfix/12048-remove-virtualbox

Original created by @intrigeri on 12048 (Redmine)

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