iotests: Use aes-128-cbc

iotests: Use aes-128-cbc

Our gnutls crypto backend (which is the default as of 8bd0931f6)
supports neither twofish-128 nor the CTR mode.  CBC and aes-128 are
supported by all of our backends (as far as I can tell), so use
aes-128-cbc in our iotests.

(We could also use e.g. aes-256-cbc, but the different key sizes would
lead to different key slot offsets and so change the reference output
more, which is why I went with aes-128.)

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211117151707.52549-2-hreitz@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>