Add a nobwshaping slot to virt_lans. Set in the front end, and used
by assign_wrapper to help control emulated links. When set for an emulated link, assign uses the BW spec for mapping the links (onto phys links) but does not insert a linkdelay to traffic shape it (unless the link is shaped in some other manner, like a delay or plr, in which case a linkdelay is inserted, but with no bw cap). This might be temporary syntax, expecially if we cannot make emulated links work right!