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: NStefan Zimmermann <stzi@linux.vnet.ibm.com>
Reviewed-by: NBoris Fiuczynski <fiuczy@linux.vnet.ibm.com>