network: support setting firewalld zone for bridge device of open networks

network: support setting firewalld zone for bridge device of open networks

The bit of code that sets the firewalld zone was previously a part of
the function networkAddFirewallRules(), which is not called for
networks with <forward mode='open'/>.

Setting the 'libvirt' zone for the bridge device of virtual networks
that also add firewall rules is usually necessary in order to get the
expected traffic through without modifying firewalld's default zone
(which would be a bad idea, because that would affect all the other
host interfaces set to the default zone), but in general we would
*not* want the bridge device for a mode='open' virtual network to be
automatically placed in the "libvirt" zone. However, a user might want
to *explicitly* set some other firewalld zone for mode='open'
networks, and libvirt's network config is a convenient place to do
that.

We enable this by moving the code that sets the firewalld zone into a
separate function that is called for all forward modes that use a
bridge device created/managed by libvirt (nat, route, isolated,
open). If no zone is specified, then the bridge device will be in
whatever zone interfaces are put in by default, but if the <bridge>
element has a "zone" attribute, then the new bridge device will be
placed in the specified zone.

NB: This function is only called when the network is started, and
*not* when the firewall rules of an active network are reloaded at
virtnetworkd restart time, because the firewalld zone of an interface
isn't something that gets inadvertantly changed as a part of some
other unrelated action. For example all iptables rules are cleared by a
firewalld restart, including those rules added by libvirt, but there
is no blanket action that changes the zone of all interfaces, so it's
useful for libvirt to reload its rules when restarting virtnetworkd,
but pointless to re-add the interface to its preferred zone.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/215
Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>