conf: add a virFirewall object to virNetworkObj

conf: add a virFirewall object to virNetworkObj

This virFirewall object will store the list of actions required to
remove the firewall that was added for the currently active instance
of the network, so it has been named "fwRemoval" (and when parsed into
XML, the <firewall> element will have the name "fwRemoval").

There are no uses of the fwRemoval object in the virNetworkObj yet,
but everything is in place to add it to the XML when formatted, parse
it from the XML when reading network status, and free the virFirewall
object when the virNetworkObj is freed.

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