Although the virtual network may be working in your KVM/QEMU instance manager, you still might not get a connection through. Here is a solution:
On your host system, change firewall_backend in /etc/libvirt/network.conf from nftables to iptables.
If you want to be extra sure in your changes, restart the iptables and libvirtd services:
sudo systemctl restart libvirtd.service
sudo systemctl restart iptables.service