Kernel: Detect devices when enumerating the PCI bus

Instead of making each driver to enumerate the PCI bus itself,
PCI::Initializer will call detect_devices() to do one enumeration
of the bus.

Kernel/PCI/Initializer.h

@@ -39,10 +39,11 @@ public:
     static void dismiss();
 
 private:
+    void detect_devices();
     ~Initializer();
     Initializer();
     bool test_acpi();
     bool test_pci_io();
     bool test_pci_mmio();
     void initialize_pci_mmio_access_after_test();
-};
+};