/etc/libvirt/nwfilter/ is in libvirt-bin 1.3.1-1ubuntu10.
This file is owned by root:root, with mode 0o755.
..
/etc/libvirt/nwfilter/allow-arp.xml
/etc/libvirt/nwfilter/allow-dhcp-server.xml
/etc/libvirt/nwfilter/allow-dhcp.xml
/etc/libvirt/nwfilter/allow-incoming-ipv4.xml
/etc/libvirt/nwfilter/allow-ipv4.xml
/etc/libvirt/nwfilter/clean-traffic.xml
/etc/libvirt/nwfilter/no-arp-ip-spoofing.xml
/etc/libvirt/nwfilter/no-arp-mac-spoofing.xml
/etc/libvirt/nwfilter/no-arp-spoofing.xml
/etc/libvirt/nwfilter/no-ip-multicast.xml
/etc/libvirt/nwfilter/no-ip-spoofing.xml
/etc/libvirt/nwfilter/no-mac-broadcast.xml
/etc/libvirt/nwfilter/no-mac-spoofing.xml
/etc/libvirt/nwfilter/no-other-l2-traffic.xml
/etc/libvirt/nwfilter/no-other-rarp-traffic.xml
/etc/libvirt/nwfilter/qemu-announce-self-rarp.xml
/etc/libvirt/nwfilter/qemu-announce-self.xml