sheep: use xmalloc instead of malloc

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep: remove nr_copies parameter

We have a vdi_copy tree and we can get the vdi copy information for the tree
on the fly, no need to get it from inode.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep: unify snapid passing and put it in vdi_iocb

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep: pass vdi_iocb for vdi deletion

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep: refactor vdi lookup, create, delete operation

This patch set only refactor vdi code of the sheep side, so try to maintain
the same behavior as old code and (should) passes all the tests regarding 
VDI operation.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
tests/039: fix the output for the refactored vdi code

Highest snapid is internally used by SD for the next snapshot, it should means
not exist for users. So we should NO_TAG if users want to access with this
invalid tag.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>