For s390 the default console target type is virtio. This also requires
that an implicit virtio-serial controller is instantiated.
This testcase verifies that the target type of virtio is correctly set
in the generated XML if no target element was given and that the
corresponding virtio-serial element is generated too.
Signed-off-by: NViktor Mihajlovski <mihajlov@linux.vnet.ibm.com>