libxl: add default controllers for USB devices
    
    Attempting to start a domain with USB hostdevs but no USB controllers
    fails with the rather cryptic error
    
    libxl: error: libxl_qmp.c:287:qmp_handle_error_response: received an
    error message from QMP server: Bus 'xenusb-0.0' not found
    
    This can be fixed by creating default USB controllers. When no USB
    controllers are defined, create the number of 8 port controllers
    necessary to accommodate the number of defined USB devices.
    
    Note that USB controllers are already created as needed in the
    domainAttachDevice code path. E.g. a USB controller will be created,
    if necessary, when attaching a USB device with
    'virsh attach-device dom usbdev.xml'.