qemu-img info: Prefix parsing functions as old_parser_*.

Prefix functions in this file with old_parser_... to make it clear
this is the old / slightly unsafe human output parser, which is
shortly to be replaced by a safer JSON parser.

This is just code motion.
qemu-img info: Use --output json and YAJL (JSON parser) to safely parse output (RHBZ#920225).

The old parser is still used if either YAJL is not available or
qemu-img info doesn't support the --output json option.