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/TTY/VirtualConsole.h

@@ -84,9 +84,11 @@ public:
 
     void emit_char(char);
 
-private:
+    // FIXME: We expose these constructors to make try_create_device helper to work
     explicit VirtualConsole(const unsigned index);
     VirtualConsole(const unsigned index, const CircularQueue<char, 16384>&);
+
+private:
     // ^KeyboardClient
     virtual void on_key_pressed(KeyEvent) override;