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/Graphics/FramebufferDevice.h

@@ -19,6 +19,8 @@ namespace Kernel {
 
 class FramebufferDevice : public BlockDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
     static NonnullRefPtr<FramebufferDevice> create(const GraphicsDevice&, size_t, PhysicalAddress, size_t, size_t, size_t);
 
@@ -32,10 +34,9 @@ public:
     virtual ~FramebufferDevice() {};
     KResult initialize();
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     FramebufferDevice(const GraphicsDevice&, size_t, PhysicalAddress, size_t, size_t, size_t);
 
-private:
     // ^File
     virtual StringView class_name() const override { return "FramebufferDevice"; }