Added Mirage support.

Please review and comment on this patch.

I think this could enable the Xs module in Mirage to be completely deteled (Apart from the IO module and the instanciation Xs = Xs_client_lwt(IO)).

I had one question about the code in Mirage. There is a mutex that protects the make () function, but then, if different parts of the code hold the client, how calls to xenstore are serialized ?