qemu: Set default SCSI controller model for S390 arch
    
    When no model is specified in the domain definition for
    a scsi controller and the architectur is s390 than virtio-scsi
    is set as default model.
    
    Signed-off-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
    Reviewed-by: Daniel Hansel <daniel.hansel@linux.vnet.ibm.com>
    Reviewed-by: Stefan Zimmermann <stzi@linux.vnet.ibm.com>
    Reviewed-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
    Reviewed-by: Michal Privoznik <mprivozn@redhat.com>