function-test/vm-disk: start writing and reviewing this document

首先是一些补全的修复, 另外头部加了一些申明的代码.

能不用tab补全吗? 用 git diff 很痛苦啊

Signed-off-by: Dunrong Huang <riegamaths@gmail.com>
function-test/vm-disk: finish adding disk

Signed-off-by: Dunrong Huang <riegamaths@gmail.com>