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 491d6c7.

Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
