libvirt installs the sysctl configuration file 'libvirtd' in '/etc/sysctl.d' but it's not sourced during boot. The reason being is that OpenRC only supports sysctl configuration files in the form '/etc/sysctl.d/*.conf'. The sysctl configuration file should be installed as '/etc/sysctl.d/libvirtd.conf'.
Fixed in libvirt-0.9.10-r4