network: move all functions manipulating iptables rules into network_iptables.c

network: move all functions manipulating iptables rules into network_iptables.c

Although initially we will add exactly the same rules for the nftables
backend, the two may (hopefully) soon diverge as we take advantage of
nftables features that weren't available in iptables. When we do that,
there will need to be a different version of these functions (currently in
bridge_driver_linux.c) for each backend:

  networkAddFirewallRules()
  networkRemoveFirewallRules()
  networkSetupPrivateChains()

Although it will mean duplicating some amount of code (with just the
function names changed) for the nftables backend, this patch moves all
of the rule-related code in the above three functions into iptables*()
functions in network_iptables.c, and changes the functions in
bridge_driver_linux.c to call the iptables*() functions. When we make
a different backend, it will only need to make equivalents of those 3
functions publicly available to the upper layer.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>