avoid use of /dev/disk/by-id in VirtualBox

/dev/disk/by-id is unstable under VirtualBox so we don't want to rely on
it.
Merge pull request #305 from aspiers/virtualbox-disk-by-id

avoid use of /dev/disk/by-id in VirtualBox
Merge branch 'release/roxy/master' into merge-roxy
Merge pull request #318 from vuntz/merge-roxy

Merge roxy branch into master