virtualbox: Fix permissions for /dev/vboxdrvu

See https://bugs.archlinux.org/task/38314 and https://www.virtualbox.org/browser/vbox/trunk/src/VBox/Installer/linux/installer-common.sh?rev=47894#L28
Merge pull request #3079 from wmertens/patch-4

virtualbox: Fix permissions for /dev/vboxdrvu