Mostly, Error ** is for returning error from the function, so the
callee sets it. However qbus_list_bus and qbus_list_dev 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.

While there, rename the functions to
qbus_error_append_bus_list_hint(), qbus_error_append_dev_list_hint().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20191205174635.18758-7-vsementsov@virtuozzo.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message replaced]
Signed-off-by: Markus Armbruster <armbru@redhat.com>