dog: fix improper typed 'ret' in 'vdi create'

'ret' is unsigned then never get < 0 even in error case, so for a wrong size, we
will proceed to create an invalid vdi.

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
update CHANGELOG.md for 'node stat'

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>