Fix USB model defaults for ppc64
    
    The condition was checking for UHCI (and OHCI for ppc64) availability so
    that it can specify the proper device instead of legacy usb.  However,
    for ppc64, we don't need to check both OHCI and UHCI, but only OHCI as
    that is the legacy default.  The condition is so big that it was just a
    matter of time when someone will make a mistake there, so let's use more
    lines so that it is visible what the condition checks for.
    
    This fixes usage of -device instead of -usb for ppc64 that supports
    pci-usb-ohci and does not support piix3-usb-uhci.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1297020
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>