libvirt "Authorization requires authentication but no agent is available" error on Fedora 18

With latest devstack on Fedora 18, nova-compute was failing to start up and giving this error:

  libvirtError: authentication failed: Authorization requires authentication but no agent is available.

In order to configure PolicyKit to allow nova-compute running as 'markmc' to connect to libvirtd, I had to do add this file to /etc/polkit-1/rules.d/50-libvirt-markmc.rules:

 polkit.addRule(function(action, subject) {
     if (action.id == "org.libvirt.unix.manage" &&
         subject.user == "markmc") {
         return polkit.Result.YES;
     }
 });