Adopt the same configuration interface as mirage-block-unix

[djs55]

mirage-block-unix now has an interface like

module Config: sig
  type t = {
    foo: bar;
    path: string;
  }
  val create ?foo -> string -> t
  val to_string: t -> string
  val of_string: string -> ...
end

val connect: ?foo -> string -> t io
val to_config: t -> Config.t
val of_config: Config.t -> t io

This is nice because you can either pass around serialised configuration records (e.g. read from a config file) or you can use the connect directly. We should adopt this style so that it becomes easier to add support for TRIM, which will require additional options.