sheepdog: change a method for detecting invalidation of inode

Current method of detecting invalidation of inode can produce bunch of
error messages if a first object of VDI doesn't exist. This patch
updates the method: reading first one byte of inode object. It can
reduce needless and confusing error messages.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>