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>