Kernel/Devices: Defer creation of SysFS component after the constructor

Instead of doing so in the constructor, let's do immediately after the
constructor, so we can safely pass a reference of a Device, so the
SysFSDeviceComponent constructor can use that object to identify whether
it's a block device or a character device.
This allows to us to not hold a device in SysFSDeviceComponent with a
RefPtr.
Also, we also call the before_removing method in both SlavePTY::unref
and File::unref, so because Device has that method being overrided, it
can ensure the device is removed always cleanly.

Kernel/Devices/HID/PS2MouseDevice.h

@@ -32,8 +32,10 @@ public:
         enable_irq();
     }
 
-protected:
+    // FIXME: We expose this constructor to make try_create_device helper to work
     explicit PS2MouseDevice(const I8042Controller&);
+
+protected:
     // ^IRQHandler
     virtual bool handle_irq(const RegisterState&) override;