qcow2-bitmaps: fix qcow2_can_store_new_dirty_bitmap

qcow2-bitmaps: fix qcow2_can_store_new_dirty_bitmap

qcow2_can_store_new_dirty_bitmap works wrong, as it considers only
bitmaps already stored in the qcow2 image and ignores persistent
BdrvDirtyBitmap objects.

So, let's instead count persistent BdrvDirtyBitmaps. We load all qcow2
bitmaps on open, so there should not be any bitmap in the image for
which we don't have BdrvDirtyBitmaps version. If it is - it's a kind of
corruption, and no reason to check for corruptions here (open() and
close() are better places for it).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20191014115126.15360-2-vsementsov@virtuozzo.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>