Kernel: Start work on USB stack

Current code can enumerate all xHCI devices and detect their type based
on the class code on device or interface descriptors.

kernel/kernel/kernel.cpp

@@ -27,6 +27,7 @@
 #include <kernel/Terminal/Serial.h>
 #include <kernel/Terminal/VirtualTTY.h>
 #include <kernel/Timer/Timer.h>
+#include <kernel/USB/USBManager.h>
 
 #include <LibInput/KeyboardLayout.h>
 
@@ -205,6 +206,8 @@ static void init2(void*)
 
 	MUST(NetworkManager::initialize());
 
+	MUST(USBManager::initialize());
+
 	// NOTE: PCI devices are the last ones to be initialized
 	//       so other devices can reserve predefined interrupts
 	PCI::PCIManager::get().initialize_devices();