SSH connection from host to guest VM

Settings -> Network -> Adapter X -> Attached to "NAT" -> Advanced -> Port Forwarding -> insert new rules -> Host Port: 2222, Guest Port: 22

Connect via ssh:

ssh -l userid -p 2222 localhost/127.0.0.1/hostIP

Instead of "NAT" mode, you can also use "Bridged" mode to let VM get its own IP, then connect directly to guestIP without port forwarding:

ssh -l userid guestIP

BTW, guest machine can be started headlessly by:

VBoxManage startvm "VM name" --type headless

or

VBoxHeadless --startvm <uuid|name>

VBoxManage startvm runs the VM directly as a background process, while VBoxHeadless can provide detailed messages of the started VM.