spapr: Simplify error handling in spapr_memory_plug()

As recommended in "qapi/error.h", add a bool return value to
spapr_add_lmbs() and spapr_add_nvdimm(), and use them instead
of local_err in spapr_memory_plug().

This allows to get rid of the error propagation overhead.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <160309734178.2739814.3488437759887793902.stgit@bahia.lan>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>