qcow2-bitmap: add qcow2_reopen_bitmaps_rw_hint()

qcow2-bitmap: add qcow2_reopen_bitmaps_rw_hint()

Add version of qcow2_reopen_bitmaps_rw, which do the same work but
also return a hint about was header updated or not. This will be
used in the following fix for bitmaps reloading after migration.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20180320170521.32152-2-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>