bitmap: Update count after a merge

We need an accurate count of the number of bits set in a bitmap
after a merge. In particular, since the merge operation short-circuits
a merge from an empty source, if you have bitmaps A, B, and C where
B started empty, then merge C into B, and B into A, an inaccurate
count meant that A did not get the contents of C.

In the worst case, we may falsely regard the bitmap as empty when
it has had new writes merged into it.

Fixes: be58721db
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20181002233314.30159-1-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>
(cherry picked from commit d1dde7149e)
*drop functional dep. on fa000f2f
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

util/hbitmap.c

@@ -754,6 +754,9 @@ bool hbitmap_merge(HBitmap *a, const HBitmap *b)
         }
     }
 
+    /* Recompute the dirty count */
+    a->count = hb_count_between(a, 0, a->size - 1);
+
     return true;
 }