qapi: add support to default value for optional members

Goal

Quoting Markus from the mailing list:

Extend the QAPI language to support optionals with a default value, i.e. absent is indistinguishable from present with the default value. Then use this for as many optionals as we can.

Technical details

To be defined.

Additional information

This is a proposal to the QAPI spec itself to have a simple way to express that an absent member defaults to a value.