server: persistent stream: constrain the functor output by the signature
server: persistent stream: constrain the functor output by the signature

Previously we had defined an output signature but not actually
constrained anything with it.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

mirage/ocaml-xenstore-server,djs55/ocaml-xenstore-server,mirage/ocaml-xenstore-server