After following the above steps, I create an entry called "unlock" in my ~/.ssh/config file on my Mac laptop. So after I boot my Ubuntu 16.04 LTS Desktop VM (using the VirtualBox CLI), then I type 'ssh unlock', and type 'unlock' when presented with the busybox prompt. I am prompted to enter a password to unlock the encrypted disk. After that, the system will boot up, then I can ssh in remotely.
Host unlock
Hostname 192.168.56.101
User root
IdentityFile /Users/ksa/.ssh/id_rsa_ubuntu_unlock
StrictHostKeyChecking no