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/Storage/RamdiskDevice.h

@@ -15,6 +15,7 @@ class RamdiskController;
 
 class RamdiskDevice final : public StorageDevice {
     friend class RamdiskController;
+    friend class DeviceManagement;
     AK_MAKE_ETERNAL
 public:
     static NonnullRefPtr<RamdiskDevice> create(const RamdiskController&, NonnullOwnPtr<Memory::Region>&& region, int major, int minor);