It's been a while... first of all, thank you Juanito for posting firewalld.tcz, it seems to work just fine.
Now when I start virt-manager it detects the firewall correctly, but there is another issue:
libvirt.libvirtError: Cannot check dnsmasq binary dnsmasq: No such file or directory
By searching around for this error, I found that the solution would be to recompile virt-manager with the "--with-dnsmasq-path" option
I'm not sure how to recompile it :-/