* Fix fd handling to work around Python's habit of closing files

* Fix fd handling to work around Python's habit of closing files
  you specify in subprocess.Popen.
* Error handling bugfixes: say except (IOError,OSError) everywhere
  rather than just one of those two; correct harmless bug in gpg key
  generation error handling.
* Make adt-virt-null work properly (VirtSubProc runs `down' with
  a single argument, so down must be sh -c and not []).
* In VirtSubProc close spurious copy of plumbing pipe, which prevents
  certain hangs during error situations.
* Xen cleanup script runs dmsetup info / dmsetup remove several times
  with some sleeps because xm destroy is not properly instantaneous.