To list your vms:

$ VBoxManage list vms

To run one in headless mode:

$ VBoxManage startvm freebsd-server --type headless

To shutdown:

$ VBoxManage controlvm freebsd-server acpipowerbutton

To get the IPv4:

$ VBoxManage guestproperty enumerate freebsd-server | awk '/IP/'

for this to work you will need the vboxguest tools pkg install virtualbox-ose-additions