to be performed in the event-thread.
To do this, we need to lock the eventlist with g_eventlock (nee g_doorlock),
since g_call_me() being called from the UP/DOWN paths will not be able to
aquire g_topology_lock.
This also means that for now these events are not referenced on any
particular consumer/provider/geom.
For UP/DOWN path use, this will not become a problem since the access()
function will make sure we drain any bio's before we dismantle.
Sponsored by: DARPA & NAI Labs.
