Use manual PCI search when making bios ram writable.

During the ram unlock phase static variables can't be written, so
don't rely on the higher level PCI searching functions.  This will
allow for future simplification of those high level search functions.

This also limits the scan for the memory locking device to the first
bus - the device should also be on the root bus.
diff --git a/src/shadow.c b/src/shadow.c
index 169decf..68c9230 100644
--- a/src/shadow.c
+++ b/src/shadow.c
@@ -9,6 +9,7 @@
 #include "pci.h" // pci_config_writeb
 #include "config.h" // CONFIG_*
 #include "pci_ids.h" // PCI_VENDOR_ID_INTEL
+#include "pci_regs.h" // PCI_VENDOR_ID
 #include "xen.h" // usingXen
 
 // On the emulators, the bios at 0xf0000 is also at 0xffff0000
@@ -94,22 +95,11 @@
     pci_config_writeb(bdf, pam0, 0x10);
 }
 
-static void i440fx_bios_make_writable(u16 bdf, void *arg)
-{
-    make_bios_writable_intel(bdf, I440FX_PAM0);
-}
-
 static void i440fx_bios_make_readonly(u16 bdf, void *arg)
 {
     make_bios_readonly_intel(bdf, I440FX_PAM0);
 }
 
-static const struct pci_device_id dram_controller_make_writable_tbl[] = {
-    PCI_DEVICE(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_82441,
-               i440fx_bios_make_writable),
-    PCI_DEVICE_END
-};
-
 static const struct pci_device_id dram_controller_make_readonly_tbl[] = {
     PCI_DEVICE(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_82441,
                i440fx_bios_make_readonly),
@@ -125,14 +115,19 @@
 
     dprintf(3, "enabling shadow ram\n");
 
-    // at this point, statically allocated variables can't be written.
-    // so stack should be used.
-
-    // Locate chip controlling ram shadowing.
-    int bdf = pci_find_init_device(dram_controller_make_writable_tbl, NULL);
-    if (bdf < 0) {
-        dprintf(1, "Unable to unlock ram - bridge not found\n");
+    // At this point, statically allocated variables can't be written,
+    // so do this search manually.
+    int bdf, max;
+    foreachbdf_in_bus(bdf, max, 0) {
+        u32 vendev = pci_config_readl(bdf, PCI_VENDOR_ID);
+        u16 vendor = vendev & 0xffff, device = vendev >> 16;
+        if (vendor == PCI_VENDOR_ID_INTEL
+            && device == PCI_DEVICE_ID_INTEL_82441) {
+            make_bios_writable_intel(bdf, I440FX_PAM0);
+            return;
+        }
     }
+    dprintf(1, "Unable to unlock ram - bridge not found\n");
 }
 
 // Make the BIOS code segment area (0xf0000) read-only.