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.