Add support for JSON pretty printing

Add support for JSON pretty printing

The monitor does not pretty-print JSON output, so that everything
will be on a single line reply. When JSON docs get large this is
quite unpleasant to read. For the future command line capabilities
query ability, huge JSON docs will be available. This needs the
ability to pretty-print.

This introduces a new API qobject_to_json_pretty() that does
a minimal indentation of list and dict members. As an example,
this makes

  {"QMP": {"version": {"micro": 50, "minor": 12, "package": "", "major": 0}, "capabilities": []}}

Output as

  {
      "QMP": {
          "version": {
              "micro": 50,
              "minor": 12,
              "package": "",
              "major": 0
          },
          "capabilities": [
          ]
      }
  }

NB: this is not turned on for the QMP monitor.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>