Kernel/Interrupts: Add sensible purposes to VirtIO and USB devices

When we enumerate the interrupt handlers, it's a good idea to show a
meaningful name to the user instead of "IRQ Handler".

Kernel/Devices/USB/UHCIController.h

@@ -28,6 +28,8 @@ public:
     static UHCIController& the();
     virtual ~UHCIController() override;
 
+    virtual const char* purpose() const override { return "UHCI"; }
+
     void reset();
     void stop();
     void start();