The fbdev driver cannot be used any more as there is nothing giving it the permissions to access /dev/fb when the Xserver is run as non-root. the virtualbox X11 driver is not used as it is not installed. It looks like it isn't installed because the virualbox guest-kmp isn't installed. This is most likely not installed as it isn't built for the currently shipped kernel. This may be a dependency issue which I'm currently trying to find out.