libxl: libxl-json format and internal functions to get / set it

libxl: libxl-json format and internal functions to get / set it

Introduce a new format in libxl userdata store called "libxl-json". This
file format contains JSON version of libxl_domain_config, generated by
libxl. Applications are not supposed to access this file directly.

Two internal functions to get and set libxl_domain_configuration
are also introduced. Also introduce a new error code to indicate
abnormal state that libxl-json config file is empty.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>