It’s possible to use unix domain socket files.
Relevant quotes from Proof-of-concept - Whonix on QEMU on Windows
start-whonix-gateway.bat:
-nic user,model=virtio-net-pci -netdev stream,addr.type=unix,addr.path=.\gateway.sock,server=on,id=whonixint -device virtio-net-pci,netdev=whonixint
start-whonix-workstation.bat:
-netdev stream,addr.type=unix,addr.path=.\gateway.sock,server=off,id=whonixint -device virtio-net-pci,netdev=whonixint
But that’s QEMU and needs to be translated to libvirt XML.
→ libvirt-dist/usr/share/libvirt-dist/xml at master · Kicksecure/libvirt-dist · GitHub
There is a related but not exact feature request:
Meanwhile it might be possible to use qemu:commandline in libvirt XML.
<qemu:commandline>
<qemu:arg value='-netdev'/>
...
</qemu:commandline>