Simply download the plop boot manager (something like plpbt-5.0.14.zip) from
plop.at.
Then unpack it, mount the contained .iso file to your Virtualbox machine in the
manager gui. In the machine's boot order, the CD must be selected to start first.
Now make sure your flash drive is attached to the machine:
Goto machine's settings, add a new filter in the "USB" section.
(If you cannot see any devices when selecting "New filter", make sure your user
is in the vboxusers group and the Virtualbox Extensions Package is installed on your host system.)
Boot your machine, and select USB in the plop boot manager menu, voila.
See the video below for a detailed step-by-step guide: