ppc: make Error **errp const where it is appropriate

ppc: make Error **errp const where it is appropriate

Mostly, Error ** is for returning error from the function, so the
callee sets it. However kvmppc_hint_smt_possible gets already filled
errp parameter. It doesn't change the pointer itself, only change the
internal state of referenced Error object. So we can make it Error
*const * errp, to stress the behavior. It will also help coccinelle
script (in future) to distinguish such cases from common errp usage.

While there, rename the function to
kvmppc_error_append_smt_possible_hint().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20191205174635.18758-8-vsementsov@virtuozzo.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message replaced]
Signed-off-by: Markus Armbruster <armbru@redhat.com>