Pad iommu with an empty slot (necessary for SunOS 4.1.4)

On the real hardware (SS-5, LX) the MMU is not padded, but aliased.
Software shouldn't use aliased addresses, neither should it crash
when it uses (on the real hardware it wouldn't). Using empty_slot
instead of aliasing can help with debugging such accesses.

Signed-off-by: Artyom Tarasenko <atar4qemu@googlemail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
