I just started to use libvirt and needed to have static ip assignment for some of the systems. Libvirt supports this but there is no documentation for this, a patch with documentation has just been send to the mailing list, but thats all (except the documentation in the *.c files ;-)
A simple example which assigns 192.168.122.2 to the host with the mac AA:BB:CC:DD:EE:FF:<network> <name>sample</name> <bridge name='intbr0' /> <ip address='192.168.122.1' netmask='255.255.255.0'> <dhcp> <range start='192.168.122.2' end='192.168.122.254' /> <host mac='AA:BB:CC:DD:EE:FF' ip='192.168.122.2' name='foobar'/> </dhcp> </ip> </network>