Kernel: Change the indexing of storage devices in IDEController class

Previously, the indexing scheme was that 0 is Primary-Master, 1 is
Primary-Slave, 2 is Secondary-Master, 3 is Secondary-Slave.

Instead of merely matching between numbers to the channel & position,
the IDEController code will try to find all available drives connected to
the two channels, then it will create a Vector with nonnull RefPtr to
them. Then we take use the given index with this Vector.

Kernel/Storage/IDEController.h

@@ -55,6 +55,7 @@ public:
 private:
     IDEController(PCI::Address address, bool force_pio);
 
+    RefPtr<StorageDevice> device_by_channel_and_position(u32 index) const;
     void initialize(bool force_pio);
     void detect_disks();