exec.c: Use subpages for large unaligned mappings

Registering a multi-page memory region that is non-page-aligned results
in a subpage from the start to the page boundary, some number of full
pages, and possibly another subpage from the last page boundary to the
end. The full pages will have a value for offset_within_region that is
not a multiple of TARGET_PAGE_SIZE. Accesses through softmmu are unable
to handle this and will segfault.

Handling full pages through subpages is not optimal, but only
non-page-aligned mappings take the penalty.
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

@@ -2305,10 +2305,15 @@ void cpu_register_physical_memory_log(MemoryRegionSection *section,
         remain.offset_within_address_space += now.size;
         remain.offset_within_region += now.size;
     }
+    while (remain.size >= TARGET_PAGE_SIZE) {
         now = remain;
+        if (remain.offset_within_region & ~TARGET_PAGE_MASK) {
+            now.size = TARGET_PAGE_SIZE;
+            register_subpage(&now);
+        } else {
             now.size &= TARGET_PAGE_MASK;
-    if (now.size) {
             register_multipage(&now);
+        }
         remain.size -= now.size;
         remain.offset_within_address_space += now.size;
         remain.offset_within_region += now.size;