cpu: don't allow negative core id

With pseries machine type a negative core-id is not managed properly:
-1 gives an inaccurate error message ("core -1 already populated"),
-2 crashes QEMU (core dump)

As it seems a negative value is invalid for any architecture,
instead of checking this in spapr_core_pre_plug() I think it's better
to check this in the generic part, core_prop_set_core_id()

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20170802103259.25940-1-lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
