Kernel: Initialize and expose SerialDevice(s) properly

I forgot that we need to also initialize SerialDevice and also to ensure
it creates a sysfs node properly. Although I had a better fix for this,
it keeps the CI happy, so for now it's more than enough :)

Kernel/Devices/SerialDevice.h

@@ -102,10 +102,12 @@ public:
         DataReady = 0x01 << 0
     };
 
+    // FIXME: We expose this constructor to make try_create_device helper to work
+    SerialDevice(IOAddress base_addr, unsigned minor);
+
 private:
     friend class PCISerialDevice;
 
-    SerialDevice(IOAddress base_addr, unsigned minor);
 
     // ^CharacterDevice
     virtual StringView class_name() const override { return "SerialDevice"; }