+#define FW_CFG_NX_STACK_ITEM "opt/ovmf/PcdSetNxForStack"

Why this is needed?

+//
+// Values we program into the PM base address registers //
+#define PIIX4_PMBA_VALUE 0xB000
+#define ICH9_PMBASE_VALUE 0x0600

They are in OvmfPkg/Include/OvmfPlatforms.h, no need to copy them over.

+VOID
+PciExBarInitialization (

+VOID
+MiscInitialization (

Cut+Paste from PlatformPei

Please refactor the code (move the functions needed to a Library?) so we
don't have multiple copies of the setup code.