Add an implementation of the QEMU cipher APIs to the gnutls
crypto backend. XTS support is only available for gnutls
version >= 3.6.8. Since ECB mode is not exposed by gnutls
APIs, we can't use the private XTS code for compatibility.

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