CentOS - Nov 2011 - Add new host to /etc/libvirt/qemu/networks/default.xml without virsh net-destroy?

Hi,

When I setup a new KVM guest and add it's mac address to the default
NAT'ted network, it would be nice to have that become active without having
to destroy the network and recreate it - as that tends to make the other
guests rather upset.

Anybody knows how to achieve that? I've tried virsh net-edit but that
doesn't seem to do anything useful :-(

BR Bent