Comment 50 for bug 1665698

To summarize:

Before 1.3.3)
- qemu executed the script
- passing: "" got to qemu and it and knew about it being a nop
- not passing anything: means qemu will run the default path /etc/qemu-ifup

Since 1.3.3)
- libvirt executes the script
- passing "": libvirt does not know "" should be a nop and might have had issues
- not passing anything: does nothing (= nop)

Since 3.1
- passing: "": libvirt now knows "" it is meant to be a nop
- not passing anything: does nothing (still a nop)

Because openstack wants libvirt/qemu to do nothing, I think with the openstack change in place it is only compat >=1.3.3 where "not passing anything" is a nop.