Kernel/VirtIO: Remove the m_class_name member

This class member was used only to determine the device type when
printing messages to the debug log. Instead, remove this class member,
and add a quick way to find the device type according to how the VirtIO
specification says to do that.

This simplifies construction of VirtIODevices a bit, because now the
constructor doesn't need to ask for a String identified with the device
type.

Kernel/Graphics/VirtIOGPU/GPU.h

@@ -77,6 +77,8 @@ public:
     void flush_dirty_rectangle(ScanoutID, Protocol::Rect const& dirty_rect, ResourceID);
 
 private:
+    virtual StringView class_name() const override { return "VirtIOGPU"; }
+
     struct Scanout {
         RefPtr<FrameBufferDevice> framebuffer;
         RefPtr<Console> console;