virtio: fix typo in comment

- Delete "@request_vqs" and "@free_vqs" comments, since
  they are no longer in struct virtio_config_ops.
- According to the macro below, "@val" should be "@v".

Signed-off-by: Chen Baozi <chenbaozi@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>