libxl: clean up namespace violation

libxl: clean up namespace violation

In 752f181f ("libxl_json: introduce parser functions for builtin types")
a bunch of parser functions were introduced. Unfortunately they
polluted the public namespace with a prefix "libxl_", while they should
be internal functions.

This patch changes the prefix to "libxl__". No functional change
introduced.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>