How thick I must have been! In my earlier blog posts with VGA passthrough succeeded, I said that I couldn't passthrough any USB device at all.
As a matter of fact, Timothy had corrected my usbdevice directive in the guest config file in an earlier mailing list post but I didn't take a careful look at the usbdevice directive at all. The correct usbdevice directive is:
usbdevice = "host:vendorid:productid"
I had used
usbdevice = [ 'host:vendorid:productid' ]
which is completely erroneous. I need to omit the square brackets for USB device passthrough to work.
I have now successfully passed through the USB optical mouse to the Windows XP Home HVM domU and using on-screen keyboard as a substitute for the physical USB keyboard.
The reason I am doing this is because QEMU with Xen can only allow one USB device to be passed through at any one time.
It was so obvious and I missed the obvious!