automation: shore up rebooting behavior There was a race condition in the old code. Use instance.stop()/instance.start() to eliminate it. As part of debugging this, I also found another race condition related to PowerShell permissions after the reboot. Unfortunately, I'm not sure the best way to work around it. I've added a comment for now. Differential Revision: https://phab.mercurial-scm.org/D6288