I think I have fixed it
fstab must look like this
usbfs /proc/bus/usb usbfs auto,busgid=X,busmode=0775,devgid=X,devmode=0664 0 0
where X is the group id of the vboxuser group. You can get it by
cat /etc/group | grep vboxusers | cut -d : -f 3
on my side I also need to manually mount the usbfs via
so I placed this line in the /etc/init.d/vboxdrv script in the start function
begin_msg "Starting VirtualBox kernel module"