For architecture "s390x",the disk and the network device are base
on "virtio" bus.The cdrom is based on "scsi".So set the default
cdrom bus as "scsi",the default bus as "virtio".Also the default
machine type is set to "s390-ccw-virtio" as it is the only supported
in "s390x".Also add a test cast of virt-install by cdrom in s390x.
(crobinso: Tweak test suite and minor formatting stuff)