network: don't "refresh" iptables rules on rule-less networks

network: don't "refresh" iptables rules on rule-less networks

The bridge driver implementation of virNetworkUpdate() removes and
re-adds iptables rules any time a network has an <ip>, <forward>, or
<forward>/<interface> element updated. There are some types of
networks that have those elements and yet have no iptables rules
associated with them, and unfortunately the functions that remove/add
iptables rules don't check the type of network before attempting to
remove/add the rules, sometimes leading to an erroneous failure of the
entire update operation.

Under normal circumstances I would refactor the lower level functions
to be more robust, but to avoid code churn as much as possible, I've
just added extra checks directly to networkUpdate().