qemuxml2argvtest: Don't use privileged mode upfront
    
    When building packages in a clean chroot the QEMU_USER and QEMU_GROUP
    don't exist making VirQemuDriverConfigNew fail with privileged=true.
    
    Avoid that by not requiring privileged mode upfront but setting it later
    so we skip the user/group existence check.
    
    This solution was suggested by Daniel P. Berrange and tested by Martin
    Kletzander.