Libvirtd not updating qemu config

I’ve put in my config

virtualisation.libvirtd.qemu.verbatimConfig = ''
  namespaces = []
  vnc_listen = "0.0.0.0"
'';

this builds with no errors, and I can verify the correct value loading the config into nix repl

yet this produces no change in /var/lib/libvirt/qemu.conf or influence libvirt/qemu in the way I’m after