Currently I am testing the new Xubuntu 17.04. It still in development version but I can run it on my Virtualbox without problem. By default, Xubuntu 17.04 is shipped with Guest Additions installed. But since I have newer version of Virtualbox on my host computer, I need to reinstall or update the guest additions on Xubuntu 17.04. Installing Guest Additions on Xubuntu 17.04 is very easy.
Steps to install Guest Additions on Xubuntu 17.04
Step 1. Boot Xubuntu 17.04
Step 2. Mount Guest Additions CD image from Virtualbox Device menu.
Step 3. Install the VBoxLinuxAdditions.run
Open Thunar File Manager and click the mounted device. You should see VBOXADDITIONS_xxxx on Thunar. See the picture below.
Once completed, simply log out and log in again. The Guest Additions now should be enabled. Enjoy.