Can virtualbox-modules have the vboxvideo and its prereq vboxsf kernel module added to the build? This is enough to get the modesetting DDX driver working on virtualbox guests Alternatively could xf86-video-virtualbox be updated to 5.1.6
app-emulation/virtualbox-modules provides the kernel modules for the virtualbox host system, not the guest. Unfortunately , until bug #582078 has been fixed there's no possibility to bump xf86-video-virtualbox.