S390: Do not generate a default USB controller

S390: Do not generate a default USB controller

For s390 we don't want to have a default USB device generated even
if QEMU is silently tolerating -usb on the command line. This may change
in the future.
Another reason to avoid the USB controller is that it implies a PCI
bus which might cause a regression at some later point in time.
The following change will set the USB controller model to 'none'
unless a model or address has been specified, which can be the case
if a legacy definition is loaded or the XML writer knows what
she/he's doing.
Requiring the user to explicitly disable USB on systems not supporting
it seems cumbersome.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>