Libvirt uses dnsmasq to provide a DHCP service for the guests that are using NAT'd networking. I have one guest running with a hostname of "kraken" and to find it's IP address.
1273825151 52:54:00:76:a9:d7 192.168.122.84 kraken *
with the second entry being the MAC address and the third being the IP addr and the fourth being the hostname. However I find it strange that this information is not available via the libvirt commands or at least I cannot find it.