qemu: Set default SCSI controller model for S390 arch

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>