Prevent default creation of usb controller on s390 and s390x
    
    Since s390 does not support usb the default creation of a usb controller
    for a domain should not occur.
    
    Also adjust s390 test cases by removing usb device instances since
    usb devices are no longer created by default for s390 the s390
    test cases need to be adjusted.
    
    Signed-off-by: Stefan Zimmermann <stzi@linux.vnet.ibm.com>
    Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>