block/nbd: Fix the leaked visitor

This patch frees the leaked visitor in nbd_refresh_filename() and uses
visit_free() to fix it. The leak was introduced by the commit 491d6c7c.
Signed-off-by: NAshijeet Acharya <ashijeetacharya@gmail.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/nbd.c

@@ -536,6 +536,7 @@ static void nbd_refresh_filename(BlockDriverState *bs, QDict *options)
     ov = qobject_output_visitor_new(&saddr_qdict);
     visit_type_SocketAddress(ov, NULL, &s->saddr, &error_abort);
     visit_complete(ov, &saddr_qdict);
+    visit_free(ov);
     assert(qobject_type(saddr_qdict) == QTYPE_QDICT);
 
     qdict_put_obj(opts, "server", saddr_qdict);