spapr: Use error_append_hint() in spapr_caps.c

spapr: Use error_append_hint() in spapr_caps.c

We have a dedicated error API for hints. Use it instead of embedding
the hint in the error message, as recommanded in the "qapi/error.h"
header file.

While here, have cap_fwnmi_apply(), which already uses
error_append_hint(), to call ERRP_GUARD() as well.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <159594297421.8262.14314530897345809924.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>