I just had a frustrating couple of hours trying to get remote desktop working for a Virtualbox guest.
First, even though the settings screen implies that you merely need to turn remote support on you must also have the virtualbox extension pack installed.
Secondly, if you get "connection refused" or similar errors from the rdesktop client then you are probably using virtualbox 5.0.40
It wasn't until I upgraded virtualbox to 5.1.16 that it just magically started working. grrr