I don't have virtualbox to test, but you need to check which extensions, if any, you have set to load "onboot" (you can do this with the apps gui).
If the Xfbdev extension is set "onboot", remove it and put Xorg-7.7 in it's place. Check that you have a minimum set of extensions "onboot" (for example Xorg-7.7, flwm, aterm and wbar) and reboot.
Alternatively, you can remove all extensions from "onboot", reboot and then load a minimum set of extensions and "startx".