Recompiling with these few changes helped and the networking drivers _seems_ to be working on macOS.
At least I got past the error I was getting initially.
I'm just now dealing with a slightly different set of problems related to libvirt sockets on macOS.
The Homebrew formula ships a launchd plist which doesn't seem to do socket activation, unlike systemd.
I worked around that by setting the socket dir path manually in the config:
unix_sock_dir = "/usr/local/var/run/libvirt"
which did make libvirtd create "libvirt-sock" in that directory as expected, but per docs there should also be a secondary read-only socket "libvirt-sock-ro"
https://libvirt.org/daemons.html#monolithic-sockets and it's not there for some reason. Is there any additional configuration needed to activate that secondary socket?
Feel free to tell me if this is getting too off-topic btw. - The big picture is that I'm trying to get libvirt working on darwin and automate RPi virtualization via Vagrant (vagrant-libvirt) and taking it step by step.