Add InstanceQuery::List reply

[Hennzau]

As mentioned in a previous discussion, the zfctl instance list command did not output anything to the terminal.

This PR adds the following table output:

+---------------+-------------+----------------+
| Instance Name | Instance ID | Instance State |
+===============+=============+================+
|               |             |                |
+---------------+-------------+----------------+

[J-Loudet]

LGTM, thanks!