qemuxml2argvtest: Don't use privileged mode upfront

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.