qapi/migration: Make migration parameter @local consistently unstable

@local is unstable in MigrationParameters, but not in
MigrationParameter.  Fix the latter.

Fixes: c9affb8e9b (qapi: add local migration parameter)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Fabiano Rosas <farosas@suse.de>