serial console on guest.

On host do:

apt-get install socat screen

then setup guest:

set up a serial port in VirtualBox. 
In the Details section for the virtual machine, click Serial Ports, check Enable Serial Port, 
set Port Mode to Host Pipe, check Create Pipe and put e.g. /tmp/serialsocket1 in Port/File Path.

start the virtual machine. Use socat to connect the domain socket created by VirtualBox to a pty.

socat UNIX-CONNECT:/tmp/serialsocket1 PTY,link=/tmp/serialsocket1-pty &

Now start screen to access the pty:

screen /tmp/serialsocket1-pty

In guest os you need to have serial console enabled.

To start guest headless:

VBoxManage startvm [vm_name] -type headless