The qcow2 block driver expects to see a valid sector size even when it
has opened the crypto layer with QCRYPTO_BLOCK_OPEN_NO_IO.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>