Introduce an object for managing firewall rulesets

Introduce an object for managing firewall rulesets

The network and nwfilter drivers both have a need to update
firewall rules. The currently share no code for interacting
with iptables / firewalld. The nwfilter driver is fairly
tied to the concept of creating shell scripts to execute
which makes it very hard to port to talk to firewalld via
DBus APIs.

This patch introduces a virFirewallPtr object which is able
to represent a complete sequence of rule changes, with the
ability to have multiple transactional checkpoints with
rollbacks. By formally separating the definition of the rules
to be applied from the mechanism used to apply them, it is
also possible to write a firewall engine that uses firewalld
DBus APIs natively instead of via the slow firewalld-cmd.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>