disable mirage-entropy on xen for now (better safe than sorry)

OCaml's `random` is a LFSR, seeded with the current time. This is very predictable, especially since services might expose uptime information. I'm very much in favor of being better safe than sorry, and enabling mirage-entropy only after we have it in a decent state on Xen. @pqwy shares this opinion I believe.

@avsm are you ok with merging?