exec.c: Fix off-by-one error in register_subpage

subpage_register() expects "end" to be the last byte in the mapping.
Registering a non-page-aligned memory region that extends up to or
beyond a page boundary causes subpage_register() to silently fail
through the (end >= PAGE_SIZE) check.

This bug does not cause noticeable problems for mappings that do not
extend to a page boundary, though they do register an extra byte.
Signed-off-by: NTyler Hall <tylerwhall@gmail.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

exec.c

@@ -2271,7 +2271,7 @@ static void register_subpage(MemoryRegionSection *section)
         subpage = container_of(existing->mr, subpage_t, iomem);
     }
     start = section->offset_within_address_space & ~TARGET_PAGE_MASK;
-    end = start + section->size;
+    end = start + section->size - 1;
     subpage_register(subpage, start, end, phys_section_add(section));
 }