Allow crypto structs to be used with g_autoptr, avoiding the need to
explicitly call XXX_free() functions when variables go out of scope on
the stack.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>