If you want to pass a USB device through to a Virtual machine, you won't want to plug the USB device directly into the host that the virtual machine is currently running on (although you can) because it will tie that virtual machine to that host. You wont be able to VMotion that VM off the host without losing connection to the USB device.
Why not just buy an off the shelf product?
Well an off the shelf Network to USB device will cost you ~£250 this will cost you ~£46 **
Examples of applications for use?