qemu: conf: Add 'deprecation_behavior' setting to qemu.conf

qemu: conf: Add 'deprecation_behavior' setting to qemu.conf

New QEMU supports a harsh, but hard to ignore way to notify that the
QMP user used a deprecated command. This is useful e.g. for developers
to see that something needs to be fixed.

This patch introduces a qemu.conf option to enable the setting in cases
when qemu supports it so that developers and continiuous integration
efforts are notified about use of deprecated fields before it's too
late.

The option is deliberately stored as string and not validated to prevent
failures when downgrading qemu or libvirt versions. While we don't
support this, the knob isn't meant for public consumption anyways.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>