util: new virFirewallD APIs + docs

util: new virFirewallD APIs + docs

virFirewallDGetBackend() reports whether firewalld is currently using
an iptables or an nftables backend.

virFirewallDGetVersion() learns the version of the firewalld running
on this system and returns it as 1000000*major + 1000*minor + micro.

virFirewallDGetZones() gets a list of all currently active firewalld
zones.

virFirewallDInterfaceSetZone() sets the firewalld zone of the given
interface.

virFirewallDZoneExists() can be used to learn whether or not a
particular zone is present and active in firewalld.

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