Host USB device passthrough to RHEV guests, by either bus and device IDs, or product/vendor IDs

Issue

• RHEV allows a client to pass through devices by product/vendor.
• There is a requirement for host USB device passthrough to libvirt guests, by either bus and device IDs, or product/vendor.
• This is required because, when using only product/vendor, there would be duplicates if the same USB device type was used. As such, we need bus and device IDs when there are duplicates, but the feature should also support product/vendor strings.
• The customer would like to plug in multiple USB dongles of the same product/vendor type.
• To be able to successfully use multiple pens of the same type with RHEV, without duplication issues arising.