cpu_physical_memory_sync_dirty_bitmap: Another alignment fix

This code has an optimised, word aligned version, and a boring
unaligned version. My commit f70d3451 fixed one alignment issue, but
there's another.

The optimised version operates on 'longs' dealing with (typically) 64
pages at a time, replacing the whole long by a 0 and counting the bits.
If the Ramblock is less than 64bits in length that long can contain bits
representing two different RAMBlocks, but the code will update the
bmap belinging to the 1st RAMBlock only while having updated the total
dirty page count for both.

This probably didn't matter prior to 6b6712ef which split the dirty
bitmap by RAMBlock, but now they're separate RAMBlocks we end up
with a count that doesn't match the state in the bitmaps.

Symptom:
  Migration showing a few dirty pages left to be sent constantly
  Seen on aarch64 and x86 with x86+ovmf
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reported-by: NWei Huang <wei@redhat.com>
Fixes: 6b6712efSigned-off-by: NPaolo Bonzini <pbonzini@redhat.com>

include/exec/ram_addr.h

@@ -391,9 +391,10 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
     uint64_t num_dirty = 0;
     unsigned long *dest = rb->bmap;
 
-    /* start address is aligned at the start of a word? */
+    /* start address and length is aligned at the start of a word? */
     if (((word * BITS_PER_LONG) << TARGET_PAGE_BITS) ==
-         (start + rb->offset)) {
+         (start + rb->offset) &&
+        !(length & ((BITS_PER_LONG << TARGET_PAGE_BITS) - 1))) {
         int k;
         int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
         unsigned long * const *src;