sheep: return error if the requested snapshot is the current vdi

It is confusing that we can specify the current vdi with snapid.  This
patch makes sheep return SD_RES_INVALID_PARMS in the such case.

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

read_vdi_obj and do_vdi_delete return exit statuses, so we must
compare their return values with EXIT_SUCCESS.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
test/039: add test of rollbacking to the non-snapshot vdi

We cannot rollback to the current vdi.  This patch adds check for it.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
collie: use snapid zero for the current vdi

Using non-zero snapid for the current vdi is confusing.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
collie: force snapshot id to be larger than zero

The snapshot with id zero does not exist.

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

This fixes a false alarm of ./check -valgrind and reduces the risk of
stack overflow.

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