nixos/virtualbox-image: Use 32MB of video memory.

Booting the demo/installer image won't work if the video memory is too
low. It boots into KDE, shows the background image and doesn't do
anything, according to @domenkozar.

Thanks to @domenkozar for reporting and testing this with 32MB.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>