After a discussion on irc I filed an upstream bug. We need to have ip6tables or it fails.
24 May 2015; Matthias Maier <tamiko@gentoo.org> libvirt-1.2.10-r4.ebuild, libvirt-1.2.10-r5.ebuild, libvirt-1.2.11-r4.ebuild, libvirt-1.2.12-r1.ebuild, libvirt-1.2.12.ebuild, libvirt-1.2.13-r2.ebuild, libvirt-1.2.14-r2.ebuild, libvirt-1.2.15.ebuild, libvirt-9999.ebuild: rdepend on iptables[ipv6], bug #550116, thanks to Agostino Sarubbo; update slot dependencies for 1.2.10-r4 and 1.2.12