A day after filing a bug report on Launch Pad, I was directed to install a new version of libvirt-bin from proposed.

     I installed and restarted libvirt-bin and guest machines did not restart so that nasty bug has been squashed.

     The version of libvirt-bin that fixes this bug is 1.2.16-2ubuntu11.15.10.3.  This is for Ubuntu 15.10.