test-qdev-global-props: Initialize not_used=true for all props

This will ensure we are actually testing the code which sets
not_used=false when the property is used.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
