sheepdog: clean up sd_aio_setup()

The last two parameters of sd_aio_setup() are never used, so remove them.

Cc: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>