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

Mostly, Error ** is for returning error from the function, so the
callee sets it. However these three functions get already filled errp
parameter. They don'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.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20191205174635.18758-4-vsementsov@virtuozzo.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message typo fixed]
Signed-off-by: Markus Armbruster <armbru@redhat.com>