libvirtd.conf: Fix invalid default of max_anonymous_clients
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1343442
    
    When a client connects, it is placed into a queue. As soon as it
    authenticate, it is taken out of that queue and placed into a
    different one. Now, we have a setting in the daemon config file
    that allows users to control the length of the queue of yet not
    authenticated clients. By default, it has a value 20 but in the
    description to the config knob we clam it's zero.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>