Change the default of mdns_adv to false

Change the default of mdns_adv to false

 * Don't advertise information on the network without consent of
   the user, either through manual configuration, or a user
   interface that drives this option.
 * Since libvirtd must be configured for network access anyway
   (for all but ssh), this setting was not useful "out of the box",
   so changing this default setting does not remove "out of the box"
   functionality.