Kernel: Introduce the DeviceManagement singleton

This singleton simplifies many aspects that we struggled with before:
1. There's no need to make derived classes of Device expose the
constructor as public anymore. The singleton is a friend of them, so he
can call the constructor. This solves the issue with try_create_device
helper neatly, hopefully for good.
2. Getting a reference of the NullDevice is now being done from this
singleton, which means that NullDevice no longer needs to use its own
singleton, and we can apply the try_create_device helper on it too :)
3. We can now defer registration completely after the Device constructor
which means the Device constructor is merely assigning the major and
minor numbers of the Device, and the try_create_device helper ensures it
calls the after_inserting method immediately after construction. This
creates a great opportunity to make registration more OOM-safe.

Kernel/Devices/HID/VMWareMouseDevice.h

@@ -17,13 +17,14 @@ namespace Kernel {
 
 class VMWareMouseDevice final : public PS2MouseDevice {
 public:
+    friend class DeviceManagement;
     static RefPtr<VMWareMouseDevice> try_to_initialize(const I8042Controller&);
     virtual ~VMWareMouseDevice() override;
 
     // ^I8042Device
     virtual void irq_handle_byte_read(u8 byte) override;
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     explicit VMWareMouseDevice(const I8042Controller&);
 };