Ubuntu to Serial Console on BBB

In the VB window where your guest is running, 
click on Devices -> USB -> Prolific Technology Inc (or whatever) USB device you want to share to your guest.
Once you do that, Ubuntu will "see" the USB connection and automatically create the necessary /dev/ttyUSB0 device.

Then you can use any of the normal tools to open up /dev/ttyUSB0.  Screen, minicom, gtkterm, etc.