Please stabilize.
It looks like app-emulation/virtualbox-5.0.32 is better candidate, because it supports kernels 4.9.x (https://www.virtualbox.org/ticket/16286?cversion=0&cnum_hist=6), which will be the next LTS kernels. I've just successfully installed virtualbox-5.0.32 with gentoo-sources-4.9.4.
Also that kernel is going to be stabilized soon
This bug is now obsolete due to bug #607674 virtualbox-5.0.32 is already stable for amd64 and x86 hopefully will follow soon.