VirtualBox: mount a shared folder automatically in Gentoo

view story

http://unix.stackexchange.com – I'm currently writing a template (in veewee) to build a vagrant box running Funtoo x86_64. Currently, my box fails the last step for vagrant basebox validate, in which a shared folder has to be mounted automatically in the guest system. What are the prerequisites for automounting of shared folders? I searched for it in the documentation for Gentoo and for VirtualBox, had a look at templates for Ubunto boxes - but still don't know how to replicate the automounting behavior of the default Ubuntu box. I'm using genkernel with the gentoo-sources. The virtualbox-guest-additions are installed and (HowTos)