sheep: remove disk write cache

This removes disk cache from 'w' option because it shows poor
performance against writethrough I/O requests because of many syncfs
calls.  Currently, object cache doesn't break block semantic against
live migration and VM shutdown, so it will be fit for most users.

This breaks backward compatibility.  The syntax will be as follows:

 $ sheep -w size=<size>[,directio][,dir=<dir>]

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>