Kernel/x86: Tweak an outdated FIXME

We do support retrieving PCIe host controllers from the devicetree
on AArch64 and RISC-V now.

The microvm devicetree doesn't seem to be intended to be used by the OS,
only for EDK II. We seem to be supposed to use an AML interpreter to
call the _CBA method.

Kernel/Arch/x86_64/PCI/Initializer.cpp

@@ -44,10 +44,11 @@ UNMAP_AFTER_INIT void initialize()
         g_pci_access_is_disabled_from_commandline.set();
 
     Optional<PhysicalAddress> possible_mcfg;
-    // FIXME: There are other arch-specific methods to find the memory range
-    // for accessing the PCI configuration space.
-    // For example, the QEMU microvm machine type might expose an FDT so we could
-    // parse it to find a PCI host bridge.
+
+    // FIXME: Support accessing the PCI configuration space via the
+    //        _CBA ACPI method. To do so, we likely also need to assign
+    //        resources specified by the _CRS method, similar to the devicetree code.
+
     if (ACPI::is_enabled()) {
         possible_mcfg = ACPI::Parser::the()->find_table("MCFG"sv);
         if ((!test_pci_io()) && (!possible_mcfg.has_value()))