libvirtd.conf: Fix invalid default of max_anonymous_clients

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>