Howdy. I'm running RHEL5.2, and libvirt 0.6.1. I don't use the distro-provided firewall (system-config-securitylevel-tui-1.6.29.1-2.1.el5) and have it completely disabled, but libvirt appears to be having some trouble ascertaining as much:

libvirtd: 14:05:42.807: warning : Failed to read /etc/sysconfig/system-config-firewall last message repeated 2 times kernel: fvtebr0: Dropping NETIF_F_UFO since no NETIF_F_HW_CSUM feature. kernel: fvtebr0: starting userspace STP failed, starting kernel STP libvirtd: 14:06:34.825: error : internal error '/usr/sbin/lokkit --nostart --custom-rules=ipv4:filter:/var/lib/libvirt/iptables/filter/INPUT.chain' exited with non-zero status 1 and signal 0: --custom-rules=ipv4:filter:/var/lib/libvirt/iptables/filter/INPUT.chain: unknown option libvirtd: 14:06:34.825: warning : Failed to run '/usr/sbin/lokkit --custom-rules=ipv4:filter:/var/lib/libvirt/iptables/filter/INPUT.chain': Invalid argument libvirtd: 14:06:34.828: error : internal error '/usr/sbin/lokkit --nostart --custom-rules=ipv4:filter:/var/lib/libvirt/iptables/filter/FORWARD.chain' exited with non-zero status 1 and signal 0: --custom-rules=ipv4:filter:/var/lib/libvirt/iptables/filter/FORWARD.chain: unknown option libvirtd: 14:06:34.828: warning : Failed to run '/usr/sbin/lokkit --custom-rules=ipv4:filter:/var/lib/libvirt/iptables/filter/FORWARD.chain': Invalid argument libvirtd: 14:06:34.831: error : internal error '/usr/sbin/lokkit --nostart --custom-rules=ipv4:nat:/var/lib/libvirt/iptables/nat/POSTROUTING.chain' exited with non-zero status 1 and signal 0: --custom-rules=ipv4:nat:/var/lib/libvirt/iptables/nat/POSTROUTING.chain: unknown option libvirtd: 14:06:34.831: warning : Failed to run '/usr/sbin/lokkit --custom-rules=ipv4:nat:/var/lib/libvirt/iptables/nat/POSTROUTING.chain': Invalid argument