[Sheepdog] [PATCH v2] abstract store IO interface

[Liu Yuan]

This is an atemp to abstract store IO interface to adopt sheepdog to more
kv-stores.

changes v2:
 - use void pointer for private data transfer.
 - open: return the return value, and the private data in the argument.
 - open: use create flag for creating new file instead of posix flags.
 
 As is sugguested by Christoph Hellwig. Thanks!

 - read/write: use xpread/xpwrite for short rw and other conditions, but
   not return SD_RES_EIO, just retrn -1 in error case as system call read/write.

Thanks,
Yuan