Firewalld breaks libvirt networking on Ubuntu 18.10

After upgrading to Kubuntu 18.10, my virtual machine was no longer able to connect to the Internet. It turned out that this problem had already been reported on Fedora:

https://bugzilla.redhat.com/show_bug.cgi?id=1623868

Once I changed the firewalld configuration (etc/firewalld/firewalld.conf) to use the iptables backend as suggested in that bug report, the virtual machine could connect to the Internet again.