It has been quite a while since I started X server in one of my development gentoo virtualbox guest. Today, when I tried to startx, I got the following error:

dlopen: /usr/lib/xorg/modules/drivers/vboxvideo_drv.so: undefined symbol: resVgaShared
(EE) Failed to load /usr/lib/xorg/modules/drivers/vboxvideo_drv.so
(EE) Failed to load module "vboxvideo" (loader failed, 7)

And X failed to start.  I tried to reinstall virtualbox-guest-additions, and rebooted, but X still exists with the same error. It turns out, in order to “properly” re-install virtualbox driver in the guest, I have to re-emerge the following two packages:

emerge x11-drivers/xf86-video-virtualbox x11-drivers/xf86-input-virtualbox

The problem is caused by upgrading xorg-server from 1.6.x to 1.7.x, after which virtualbox guest drivers have to be re-installed (portage does not show a warning, so it took me some time to figure that out).