Shrink Virtualbox dynamic-allocated disk image

Follow the instructions noted in below link:

http://dantwining.co.uk/2011/07/18/how-to-shrink-a-dynamically-expanding-guest-virtualbox-image/

However, the remount command I used is: mount -no remount,ro /dev/sda1

Basically the idea is that we need to use zerofree to make all free blocks as zero, and the VBoxManager will compact it for reducing the disk size.