arch: Mark the PCI MMCONFIG region as reserved in the E820 tables

The PCI Express Firmware specification says that the region may
be included in the E820 tables (but it must always be in the ACPI
tables.)

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

arch/src/x86_64/mod.rs

@@ -22,6 +22,7 @@ use vm_memory::{
 };
 
 const E820_RAM: u32 = 1;
+const E820_RESERVED: u32 = 2;
 
 // This is a workaround to the Rust enforcement specifying that any implementation of a foreign
 // trait (in this case `DataInit`) where:
@@ -162,6 +163,13 @@ pub fn configure_system(
         }
     }
 
+    add_e820_entry(
+        &mut params.0,
+        layout::PCI_MMCONFIG_START.0,
+        layout::PCI_MMCONFIG_SIZE,
+        E820_RESERVED,
+    )?;
+
     #[cfg(feature = "acpi")]
     {
         let start_of_device_area = if mem_end < layout::MEM_32BIT_RESERVED_START {