[libvirt] [PATCH]: Allow libvirt to manage bridges "plugged into" physical devices

All, For ovirt, we need the ability to have a bridge configured that is "plugged in" to an external interface; that is, the physical interface is one of the interfaces on the bridge. This allows us to manage physical hardware outside this box, since the ovirt WUI appliance will be hooked to this same bridge and will send/receive traffic to these external machines. Currently we are doing this "by hand" with scripts, which is clearly sub-optimal. This relatively simple patch adds a new "forward" type called "bridge" (yes, it's a bad name; I'm open to suggestions). Basically, when you have a bridge with this forward type, we take the "dev" that is specified (say, eth1), plug it into the bridge, and add the appropriate iptables rule to bridge traffic. With this in place, we can get rid of our hacky scripts and let libvirt do the dirty work for us. I also imagine this could be useful to support "xen-style" bridges, without necessarily using the Xen networking scripts. Comments? Signed-off-by: Chris Lalancette <clalance(a)redhat.com&gt;