Disable Unity edge effects when running VirtualBox in fullscreen

I'm running Windows 8 from a VirtualBox machine inside Ubuntu 13.04, and I'm having some problems navigating in the virtual machine.

Many of the navigation options in Windows 8 (the new start menu, for instance) are accessible by moving the cursor to various corners and edges of the screen - this conflicts with my Unity desktop, in which I've mapped the corners to stuff as well using the Ubunut Tweak tool.

Is there any way to tell Unity and/or VirtualBox that when I run a virtual machine in full screen, I want these events forwarded to the guest OS, and no action on the host?

This behavior applies also to the Windows button (between left Alt and Fn on my keyboard), which triggers the Unity launcher instead of the Start menu, even when in fullscreen.