qapi: fix memory leak in bdrv_image_info_specific_dump

The 'obj' result of the visitor was not properly freed, like done in
other places doing a similar job.

Signed-off-by: Pino Toscano <ptoscano@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
