managed.

* Try much harder to prevent broken VMs from getting left behind.

* Try even harder to prevent left of state (devices, volumes, etc)
  from getting left behind once a VM breaks.

* Store all the assorted bits of state we create in a file so that it
  it can be cleaned up later.

* Watch for "stale" VMs that got left behind, using the UUID, which is
  regenerated each time an experiment is swapped in. Since we now
  store all the state that was associated with the stale VM, we can
  actually tear it down later. To do this by hand:

   node> sudo /usr/local/etc/emulab/mkvnode.pl -c pcvmXXX-YYY

* Lots of changes to signal handling to avoid a common problem case;
  trying to reboot or halt a VM that is still setting up.