Comment 1 for bug 252105

This is probably because you've installed the wrong flavor of the virtualbox-ose modules. You probably need virtualbox-ose-modules-generic; you're booting a linux-386 kernel probably, but without the restricted nvidia modules/drivers.
I'm marking this as duplicate of 226960, which should include a workaround/fix for this (by removing the linux-386 kernel image and/or just booting the -generic kernel).