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/Graphics/VirtIOGPU/GPU.cpp

@@ -64,6 +64,7 @@ void GPU::create_framebuffer_devices()
     for (size_t i = 0; i < min(m_num_scanouts, VIRTIO_GPU_MAX_SCANOUTS); i++) {
         auto& scanout = m_scanouts[i];
         scanout.framebuffer = adopt_ref(*new VirtIOGPU::FrameBufferDevice(*this, i));
+        scanout.framebuffer->after_inserting();
         scanout.console = Kernel::Graphics::VirtIOGPU::Console::initialize(scanout.framebuffer);
     }
 }