Sorry for the unintended reboot around 11:45AM today.  There was an update to libvirtd and every time Ubuntu updates it, it restarts it.  Restarting it is NOT supposed to interrupt service per the manual:

       Restarting libvirtd does not impact running guests.  Guests continue to
       operate and will be picked up automatically if their XML configuration
       has been defined.  Any guests whose XML configuration has not been
       defined will be lost from the configuration.

     But unfortunately, for about half the guest machines it does, regardless of what the manual states.