Introduce a simple API for handling JSON data

Introduce a simple API for handling JSON data

This introduces simple API for handling JSON data. There is
an internal data structure 'virJSONValuePtr' which stores a
arbitrary nested JSON value (number, string, array, object,
nul, etc).  There are APIs for constructing/querying objects
and APIs for parsing/formatting string formatted JSON data.

This uses the YAJL library for parsing/formatting from

 http://lloyd.github.com/yajl/

* src/util/json.h, src/util/json.c: Data structures and APIs
  for representing JSON data, and parsing/formatting it
* configure.in: Add check for yajl library
* libvirt.spec.in: Add build requires for yajl
* src/Makefile.am: Add json.c/h
* src/libvirt_private.syms: Export JSON symbols to drivers