The 's390-virtio' machine was removed from qemu in the 2.6 release.
Use the more modern s390-ccw-virtio machine type and use
VIR_TEST_CAPS_ARCH_LATEST to invoke it.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>