nixos/virtualbox: Fix setuid wrappers.

We only need to have setuid-root wrappers for VBox{Headless,SDL} and
VirtualBox, otherwise VBoxManage will run as root and NOT drop
privileges!

Fixes #5283.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
(cherry picked from commit 0d71ec8a6e538943df67e43c8b1aa667eb7cf856)