There is a file collision in: /usr/lib64/xorg/modules/drivers/vboxvideo_drv.so which belongs to x11-drivers/xf86-video-virtualbox-5.1.30
Furthermore, `=app-emulation/virtualbox-guest-additions-5.2.4` depends on both `x11-dirvers/xf86-video-virtualbox` and `x11-drivers/xf86-video-vboxvideo` (if the `X` use flag is present).
My last comment is not true. I just removed the `xf86-video-virtualbox` and I was able to update the `virtualbox-guest-additions`.
Please attach the original build log (if you still have it).
http://www.portagefilelist.de lists the packages x11-drivers/xf86-video-vboxvideo and x11-drivers/xf86-video-virtualbox for /usr/lib64/xorg/modules/drivers/vboxvideo_drv.so
The logs are still missing. Please recompile and *attach* the logfiles, https://wiki.gentoo.org/wiki/Attach_the_logs_to_the_bug_ticket and reopen this ticket (Status:unconfirmed).