exec: do not clamp accesses to MMIO regions

It is common for MMIO registers to overlap, for example a 4 byte register
at 0xcf8 (totally random choice... :)) and a 1 byte register at 0xcf9.
If these registers are implemented via separate MemoryRegions, it is
wrong to clamp the accesses as the value written would be truncated.

Hence for these regions the effects of commit 23820dbf (exec: Respect
as_translate_internal length clamp, 2015-03-16, previously applied as
commit c3c1bb99) must be skipped.
Tested-by: NHervé Poussineau <hpoussin@reactos.org>
Tested-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -341,6 +341,7 @@ address_space_translate_internal(AddressSpaceDispatch *d, hwaddr addr, hwaddr *x
                                  hwaddr *plen, bool resolve_subpage)
 {
     MemoryRegionSection *section;
+    MemoryRegion *mr;
     Int128 diff;
 
     section = address_space_lookup_region(d, addr, resolve_subpage);
@@ -350,8 +351,11 @@ address_space_translate_internal(AddressSpaceDispatch *d, hwaddr addr, hwaddr *x
     /* Compute offset within MemoryRegion */
     *xlat = addr + section->offset_within_region;
 
-    diff = int128_sub(section->mr->size, int128_make64(addr));
-    *plen = int128_get64(int128_min(diff, int128_make64(*plen)));
+    mr = section->mr;
+    if (memory_region_is_ram(mr)) {
+        diff = int128_sub(mr->size, int128_make64(addr));
+        *plen = int128_get64(int128_min(diff, int128_make64(*plen)));
+    }
     return section;
 }