nixos/virtualbox-image: Enable PAE on 32bit.

pkgs/os-specific/linux/kernel/common-config.nix defines HIGHMEM64G on
line 441 for 32bit systems, which implies PAE.

We now creating the OVA with PAE support enabled, which fixes bootup of
the image if people are just importing it without setting PAE
explicitly.

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