qcow2-refcount: Sanitize refcount table entry When reading the refcount table entry in get_refcount(), only bits which are actually significant for the refcount block offset should be taken into account. Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>