qemu: change the logic of setting default USB controller
    
    The new logic will set the piix3-uhci if available regardless of
    any architecture and it will be updated to better model based on
    architecture and device existence.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
    Acked-by: Andrea Bolognani <abologna@redhat.com>