Virt-manager (Error starting domain: Requested operation is not valid: network 'default' is not active) and what's the equivalent to (sudo virsh net-autostart --network default)

To me sounds like according to my understanding of the article one needed to set virtualisation.libvirtd.allowedBridges to what is deemed “default” (“virbr0” in the article again).

Some links around here discussing the bridge interface workings:

HTH