This script will graceful power down all VM and wait for 30 seconds. After that it will kill remaining VM that still not powerdown.

shutdown-all-vbox-vm.sh

#!/bin/bash

row=`vboxmanage list runningvms | wc -l`
col=0

# Send ACPI shutdown first
while read -r name; do
  echo "Stopping VM $(echo $name |cut -d \{ -f 1)..."
  VBoxManage controlvm "$(echo $name |cut -d \{ -f 2 |sed 's/.$//')" acpipowerbutton &> /dev/null
done < <(VBoxManage list runningvms)

msg="wait for${1}... "
        #clear
        tput cup $row $col
        echo -n "$msg"
        l=${#msg}
        l=$(( l+$col ))
for i in {30..1}
    do
        tput cup $row $l
        echo -n "$i seconds"
        sleep 1
    done

list_runningvms=`vboxmanage list runningvms | wc -l`
echo
if [ $list_runningvms -ne 0 ];
then
    # Read all pids for "VBoxHeadless" process, and kill (-9) them.
    echo "Not all VM is poweroff. Force killing.."
    while read -r pid; do
    echo "Killing VirtualBox Process '${pid}'..."
    kill -9 ${pid}
    echo "  OK Done."
    done <  <(pgrep VBoxHeadless)
else
    echo "All VM gracefully poweroff. no need force killing."
fi