fix: boot failure on VirtualBox legacy-BIOS VMs (AC'97 probed as HDA)

VirtualBox default VMs expose an AC'97 audio controller (8086:2415,
class 04/subclass 01) whose BAR0 is an I/O-space BAR. The IntelHDA
driver matched any Intel multimedia subclass, and ReadBar0 masked the
I/O BAR's port number into a bogus 'physical address' that MapMMIO
panicked on (non-page-aligned), halting boot.

Two fixes:
- IntelHDA now matches only subclass 0x03 (HD Audio); AC'97 (0x01) is
  a different programming model and is not claimed.
- ReadBar0 returns 0 (no usable MMIO BAR) for I/O-space BARs instead
  of handing a port number to MapMMIO.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

kernel/src/Drivers/Init.cpp

@@ -149,13 +149,15 @@ namespace Drivers {
             ProbeNvme,
         },
         // Order 7: Intel HDA — Normal phase, match vendor=0x8086 + class=0x04 (Multimedia)
-        //          SubClass 0x01 = "Multimedia audio controller" (most Intel HDA)
-        //          SubClass 0x03 = "Audio device" (HDA-compatible)
+        //          SubClass 0x03 = "Audio device" = HD Audio controller.
+        //          SubClass 0x01 = "Multimedia audio controller" = legacy AC'97
+        //          (e.g. VirtualBox 8086:2415), which is NOT HDA and has an I/O
+        //          BAR, not an MMIO BAR — do NOT match it here.
         {
             "IntelHDA",
             0x8086,                         // VendorId (Intel)
             0x04,                           // ClassCode (Multimedia)
-            0xFF,                           // SubClass (any — covers both 0x01 and 0x03)
+            0x03,                           // SubClass (0x03 = HD Audio; excludes AC'97 0x01)
             0xFF,                           // ProgIf (any)
             nullptr,
             0,

kernel/src/Pci/Pci.cpp

@@ -404,6 +404,14 @@ namespace Pci {
 
     uint64_t ReadBar0(uint8_t bus, uint8_t device, uint8_t function) {
         uint32_t bar0Low = LegacyRead32(bus, device, function, (uint8_t)PCI_REG_BAR0);
+
+        // Bit 0 == 1 means this is an I/O-space BAR, not memory. The low bits of
+        // an I/O BAR are a port number, not a physical address, and must never be
+        // fed to MapMMIO (it would panic on the non-page-aligned value). Callers
+        // treat 0 as "no usable MMIO BAR". (e.g. VirtualBox AC'97 8086:2415.)
+        if (bar0Low & 0x1)
+            return 0;
+
         uint64_t addr = bar0Low & 0xFFFFFFF0u;
 
         // Check for 64-bit BAR (type field bits 2:1 == 0b10)