libxl: ocaml: support for KeyedUnion in the bindings generator.

libxl: ocaml: support for KeyedUnion in the bindings generator.

A KeyedUnion consists of two fields in the containing struct. First an
enum field ("e") used as a descriminator and second a union ("u")
containing potentially anonymous structs associated with each enum
value.

We map the anonymous structs to structs named after the descriminator
field ("e") and the specific enum values. We then declare an ocaml
variant type name e__union mapping each enum value to its associated
struct.

So given IDL:

foo = Enumeration("foo", [
    (0, "BAR"),
    (1, "BAZ"),
])
s = Struct("s", [
    ("u", KeyedUnion(none, foo, "blargle", [
        ("bar", Struct(...xxx...)),
        ("baz", Struct(...yyy...)),
    ])),
])

We generate C:

enum foo { BAR, BAZ };
struct s {
    enum foo blargle;
    union {
        struct { ...xxx... } bar;
        struct { ...yyy... } baz;
    } u;
}

and map this to ocaml

type foo = BAR | BAZ;

module S = struct

    type blargle_bar = ...xxx...;

    type blargle_baz = ...yyy...;

    type blargle__union = Bar of blargle_bar | Baz of blargle_baz;

    type t =
    {
        blargle : blargle__union;
    }
end

These type names are OK because they are already within the namespace
associated with the struct "s".

If the struct associated with bar is empty then we don't bother with
blargle_bar of "of blargle_bar".

No actually change in the generated code since we don't generate any
KeyedUnions yet.

The actual implementation was inspired by
http://www.linux-nantes.org/~fmonnier/ocaml/ocaml-wrapping-c.php#ref_constvrnt

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>