GPU passthough in Vbox i think is still experimental and it is not really supported. You need to use more involved methods like KVM and XEN virtual machines.
I don't think the APUs have pass-through support (AMD-Vi, IOMMU). Is there any option for enabling an IOMMU in the BIOS? You might need other hardware mate.
Edit: I remember there was some issue trying to passthrough the graphics part of APUs. There is something called KVMGT, but I think that is only for Intel?
True but unless there is a feature needed the driver is open source and fingers cross someone can get it working if it doesn't out of the gate. One can only hope.