Comment 3 for bug 988589

Thanks for reporting, but this is not a bug if you are running Ubuntu as a VM inside Virtualbox, since Virtualbox only uses the basic VESA driver by default. You must install Virtualbox's Guest additions to access higher resolutions.

  "While the virtual graphics card which VirtualBox emulates for any guest operating system provides all the basic features, the custom video drivers that are installed with the Guest Additions provide you with extra high and non-standard video modes as well as accelerated video performance." -- https://www.virtualbox.org/manual/ch04.html