qom: Rename qdev_get_type() to object_get_type()

Commit 2f262e06 lifted qdev_get_type() from qdev to object without
renaming it accordingly.  Do that now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200707160613.848843-23-armbru@redhat.com>