admin/vm-disk: start writing this documentation

第五章请 huntxu 或者 yanying 帮忙完成, 实在讨厌权限方面的东东.

Signed-off-by: Dunrong Huang <riegamaths@gmail.com>
admin/vm-disk: finish 11.1 and 11.2

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