qemu: fix CVE-2020-27821

memory: clamp cached translation in case it points to an MMIO region

Signed-off-by: Sakib Sajal <sakib.sajal@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit df92b335)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>