and virt_lans to determine which links/lans need to be dropped out of
the list given to assign (.top file). Basically, if a simple link has
virtual node in it, get rid of the link (both sides). If a lan has a
virtual node in it, then get rid of just that member of the lan (the
other regular members need to be allocated as usual). There is a minor
problem of a lan with just one physical member left after dropping the
virtual members; not sure what to do about that so I punted on it for
now.

Note that I *do* send the virt_nodes into assign for allocation as
unconnected nodes. Its up to the user to create the routes properly,
although maybe we have to do that automatically?