Portage ignores the --jobs flag when app-emulation/virtualbox-ose is emerged on a system that has not had it previously. I suspect this is because app-emulation/virtualbox-ose-additions-3.1.2 is marked as an "interactive" package. I encountered this issue on an ~x86 system.
You might be interested in the --accept-properties=-interactive option. *** This bug has been marked as a duplicate of bug 288202 ***