vbox-guest: Remove all references to sbin/.

Using $storepath/sbin is deprecated according to commit 98cedb3d, so
let's avoid putting anything in .../sbin for the guest additions.

This is a continuation of the initial commit done by @ctheune at
1fb1360a

, which unfortunately broke VM tests and only changed the path of
the mount.vboxsf helper.

With this commit, the VM test is fixed and I've also verified on my
machine that it is indeed working again.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>