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/SerialDevice.h

@@ -13,6 +13,8 @@ namespace Kernel {
 
 class SerialDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
     static NonnullRefPtr<SerialDevice> must_create(size_t com_number);
 
@@ -102,10 +104,9 @@ public:
         DataReady = 0x01 << 0
     };
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     SerialDevice(IOAddress base_addr, unsigned minor);
 
-private:
     friend class PCISerialDevice;
 
     // ^CharacterDevice