Couple of fixes for XEN.
Couple of fixes for XEN.

1. The backend process that starts the XEN vm (xl/xm) was not happy
   with the signal mask settings. By pass that for the vnodeboot call
   so that xl/xm sees a clean mask. The sympton was processes getting
   left behind.

2. Watch for a XEN container being rebooted from inside, by waiting a
   little bit to see if comes back on its own. We used to fold right
   away, but now keep running.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome