Comment # 18 on bug 1158817 from Neil Rickert

Yes, "libvirtd" is running, and "virt-manager" is running.

As far as I know, I am not using NAT for virtual machines.

Yes, my home router provides NAT for the home network.  And the virtual
machines are assigned NAT addresses directly by the home router.  But I have
not set up NAT for virtual machines.  As far as I know, they are directly using
the bridge.

I have not attempted that firewall change, until whether there is any
clarification related to NAT.