shrink a virtualbox disk guest to free space on host

gistfile1.txt

1. Verify it is a Dynamic Disk || convert it to dynamic 
2. zerotise empty space:
  windows guest: (tested)
    2.1 download sdelete.exe (https://docs.microsoft.com/en-us/sysinternals/downloads/sdelete)
    2.2 as administrator, run: sdelete.exe c: -z
  linux guest: (untested)
    2.1 install zerofree
    2.2 reboot into recovery mode
    2.3 drop to root shell
    2.4 zerofree -v /dev/sdXY
3. VBoxManage modifymedium disk "/PATH/TO/disk.vdi" --compact