How to start a VM in headless mode from the VirtualBox GUI

I just solved this problem by clicking the green arrow "Show" at the top of the VirtualBox GUI while my headless instance was already running.

I could see what the VM window would look like inside the "Preview" Pane on the VirtualBox GUI, and after clicking "Show", the VM window came up as though I had initially powered on the machine from the Virtualbox GUI.

Edit: When you close the VM window, it will give you the additional option to "Continue running in the background", so your headless instance can revert to being headless again.

This is what I use. It starts the VMs on boot and saves their state on shutdown/reboot

#!/bin/bash
### BEGIN INIT INFO
# Provides:       vmboot
# Required-Start: vboxdrv
# Required-Stop:
# Default-Start:  2 3 4 5
# Default-Stop:   0 1 6
# Short-Description: Stop/Start VMs on System shutdown
### END INIT INFO

VBOXUSER=vboxuser
SU="sudo -H -u $VBOXUSER"
VBOXMANAGE=/usr/bin/VBoxManage
VBOXHEADLESS=/usr/bin/VBoxHeadless
RUNNINGVMS=$($SU $VBOXMANAGE --nologo list runningvms | sed -e 's/^".*".*{\(.*\)}/\1/')
ALLVMS=$($SU $VBOXMANAGE --nologo list vms | sed -e 's/^".*".*{\(.*\)}/\1/')

case $1 in
stop)
if [[ -n $RUNNINGVMS ]]; then
echo "Saving the state of all running VMs..."
for v in $RUNNINGVMS; do
    $SU $VBOXMANAGE --nologo controlvm $v savestate
done
fi
;;
start)
for v in $ALLVMS; do
if [[ -n $($SU $VBOXMANAGE --nologo showvminfo $v | grep saved) ]]; then
    echo "Restoring VMs..." && $SU $VBOXHEADLESS -s $v & > /dev/null
fi
done
;;
*)
echo "Usage: /etc/init.d/vmboot start | stop"; exit 1
;;
esac
exit 0

Just save it in /etc/init.d. I named mine vbox. Run update-rc.d <script name> defaults and you should be good to go.