Does our interface compatibility promise cover defaults?

[Markus Armbruster <armbru@redhat.com>]

From about/deprecated.rst:

    In general features are intended to be supported indefinitely once
    introduced into QEMU. In the event that a feature needs to be removed,
    it will be listed in this section. The feature will remain functional for the
    release in which it was deprecated and one further release. After these two
    releases, the feature is liable to be removed. Deprecated features may also
    generate warnings on the console when QEMU starts up, or if activated via a
    monitor command, however, this is not a mandatory requirement.

This obviously applies to syntax and semantics of our external
interface.

Does it apply to default values there?

If no: does this mean we can change defaults without notice?

If yes: does this mean any change of defaults needs notice in
about/deprecated.rst and the grace period?

Note that changing a default is a silent change, like changing semantics
/ behavior, unlike changing syntax.