Xen: generate entropy seed from /dev/urandom

The seed is stored as an OCaml string in the file `xen_seed__.ml`.  The
module `Xen_seed__` is passed as an argument to instantiate
`Entropy_xen.Make(-)`.

The seed is taken to be of random length between 2048 and 6144 bytes.
I don't know if this is a suitable size.