sheepdog: fix data format for sheepdog request and response

Data format of sheepdog backing store is different from that of Sheepdog.
This patch fixes data format of sheepdog request and response.

Signed-off-by: Teruaki Ishizaki <ishizaki.teruaki@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>