spapr: add pre_plug function for memory

This allows to manage errors before the memory
has started to be hotplugged. We already have
the function for the CPU cores.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
[dwg: Fixed a couple of style nits]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>