udev: ignore usbip-connected USB devices

Those devices are most likely attached using "PV USB" from another
domain, so it doesn't make sense to list them as available for further
passthrough.

QubesOS/qubes-issues#531

udev/udev-usb-add-change

@@ -33,6 +33,12 @@ else
 	exit 0
 fi
 
+# ignore usbip-connected devices, as most likely already passed through from
+# another VM
+if [ "`echo $DEVPATH | cut -d / -f 2-4`" = "devices/platform/vhci_hcd" ]; then
+    exit 0
+fi
+
 QDB_KEY="/qubes-usb-devices/$XSNAME"
 
 qubesdb-write "$QDB_KEY/desc" "$DESC"