spapr: Don't allow unplug of NVLink2 devices

Currently, we can't properly handle unplug of NVLink2 devices, because we
don't have code to tear down their special memory resources.  There's not
a lot of impetus to implement that: since hardware NVLink2 devices can't
be hot unplugged, the guest side drivers don't usually support unplug
anyway.

Therefore, simply prevent unplug of NVLink2 devices.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>