hbitmap: fix hbitmap_status() return value for first dirty bit case

The last return statement should return true, as we already evaluated that
start == next_dirty

Also, fix hbitmap_status() description in header

Cc: qemu-stable@nongnu.org
Fixes: a6426475a75 ("block/dirty-bitmap: introduce bdrv_dirty_bitmap_status()")
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
Message-Id: <20230202181523.423131-1-andrey.zhadchenko@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>