sheepdog: support for variable object size

Previously, Sheepdog VDI's object size was fixed to 4MB.
This patch enables to handle variable object size of Sheepdog VDI.

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