After the libvirt VM is installed, it boots again into the PXE, which thankfully then defaults to boot from local disk.
After successful deployment/provisioning, foreman should change the boot order in libvirt for the VM to remove the NIC item.
(In my setup it has also created the VM. If one would do the "create VM, copy paste mac address into interface config, might be a different story).
(or do I expect too much. Is changing the tftp files all it does? That part is working.).