inspector: Generate language bindings for OCaml.

inspector: Generate language bindings for OCaml.

This commit adds a generic mechanism for deriving language bindings
for virt-inspector, and implements one concrete binding, for OCaml.

The bindings are generated from the RELAX NG schema (virt-inspector.rng)
which is supposed to be a correct and always up to date description
of the XML that the virt-inspector program can generate.

From the RNG we generate a set of types to describe the output of
virt-inspector for the language, plus an XML parser, plus some
glue code to actually run an external instance of virt-inspector
and parse the resulting XML.

At runtime, an external 'virt-inspector --xml <name>' command runs
and the XML is parsed into language-specific structures.

This has been tested on the four example files (inspector/example?.xml)

The only particular difficulty about the OCaml binding is the use of
Obj.magic, which is naughty but works because of the isomorphism
between the representation of tuples and records in OCaml.  This
seems to cause no problems in my test program.  Apart from this, the
OCaml binding is straightforward and could be adapted easily for any
other languages that want type-safe virt-inspector bindings.

It's important to keep virt-inspector.rng up to date with changes
to virt-inspector's XML output format.