exec: Make memory_region_section_get_iotlb use section AS

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>

exec.c

diff --git a/exec.c b/exec.c
index f2e66f3..122e6e5 100644
--- a/exec.c
+++ b/exec.c
@@ -796,7 +796,7 @@
             iotlb |= PHYS_SECTION_ROM;
         }
     } else {
-        iotlb = section - address_space_memory.dispatch->map.sections;
+        iotlb = section - section->address_space->dispatch->map.sections;
         iotlb += xlat;
     }