If we have use_hotplug_scripts=false, then write libxl/disable_udev to disable the system scripts
Run the hotplug scripts from xenopsd, like libxl would.

Note in the VIF case we wait for the backend to switch to state=2
by which time the backend vif device should have been created.

Note we've added the same 'hack' to the vif script to prevent
the script running twice.