qjson: Convert to parser to recursive descent

We backtrack in parse_value(), even though JSON is LL(1) and thus can
be parsed by straightforward recursive descent.  Do exactly that.

Based on an almost-correct patch from Paolo Bonzini.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1448486613-17634-10-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>