network: allow configuring firewalld zone for virtual network bridge device

network: allow configuring firewalld zone for virtual network bridge device

Since we're setting the zone anyway, it will be useful to allow
setting a different (custom) zone for each network. This will be done
by adding a "zone" attribute to the "bridge" element, e.g.:

   ...
   <bridge name='virbr0' zone='myzone'/>
   ...

If a zone is specified in the config and it can't be honored, this
will be an error.

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