This code is preventing the MMU debug code from displaying virtual
mappings of IO devices (anything that is not located in the RAM).

Before this patch, Qemu would output 0xffffffffffffffff (-1) as the
physical address corresponding to an IO device virtual address.

With this patch the intended physical address is displayed.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>