[libvirt] networking

Hello, I'm having trouble digging up some documentation on libvirt's network config. I'm not sure where to set this up, but I have a bridged device br0 that I would like to have available in virt-manager/virsh. Right now, I can edit the VM's xml interface element manually to use br0. Virt-manager has an option for shared physical device, but it's empty, and I can't figure out how to see it from virsh. thanks -jim