block/qcow2-bitmap: fix and improve qcow2_reopen_bitmaps_rw

block/qcow2-bitmap: fix and improve qcow2_reopen_bitmaps_rw

- Correct check for write access to file child, and in correct place
  (only if we want to write).
- Support reopen rw -> rw (which will be used in following commit),
  for example, !bdrv_dirty_bitmap_readonly() is not a corruption if
  bitmap is marked IN_USE in the image.
- Consider unexpected bitmap as a corruption and check other
  combinations of in-image and in-RAM bitmaps.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190927122355.7344-9-vsementsov@virtuozzo.com
Signed-off-by: John Snow <jsnow@redhat.com>