libvirt users - May 2012 - USB passthrough: flexible configuration in XML-file needed

Hi,

I need a flexible USB passthrough of a USB-device. If the device is turned off
by the current switch (230Volt) the VM should start and not claim that the 
USB device is left. 

Using: 

 <devices>
    ?
    <hostdev mode='subsystem' type='usb'>
      <source>
        <vendor id='0x152a'/>
        <product id='0x8180'/>
      </source>
    </hostdev>
  </devices>
</domain>


in libvirt XML-config file the boot process is stopped if the device is switched
off. How can I configure the xml-file to make ich more flexible? 

Another possibility would be to passthrough USB-Slot No1 (or No 2, ...) to 
the VM. So an arbitrary USB-device that is plugged in to USB-Slot1 
would be recognized by the VM. In another forum I got the hint to 
start the VM by: 

-usb -usbdevice host:0.* -usbdevice host:1.* 

to passthrough USB-Slot No1 and No 2. How can I configure that in libvirt 
xml-file? 

tia
Eckard