* qemud/iptables.[ch]: add code for managing iptables
        rules.

        * qemud/Makefile.am: add iptables.[ch].

        * qemud/qemud.c: add and remove iptables rules as
        appropriate.

        * qemud/conf.c: when starting a guess, add a rule
        allowing it to forward packets across the networks
        bridge.

        * qemud/internal.h: add iptables context ptr

        * configure.in: add --with-iptables-dir and
        --with-iptables-prefix to allow us to put our rules
        in a chain with the given prefix and save the rules
        in files in the given dir so as to integrate with
        the proposed "service iptables restart" solution
        in:

           https://bugzilla.redhat.com/227011