crypto: wire up XTS mode for cipher APIs

Introduce 'XTS' as a permitted mode for the cipher APIs.
With XTS the key provided must be twice the size of the
key normally required for any given algorithm. This is
because the key will be split into two pieces for use
in XTS mode.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>