arch: Add SubRegion memory type

We want to be able to differentiate between memory regions that must be
managed separately from the main address space (e.g. the 32-bit memory
hole) and ones that are reserved (i.e. from which we don't want to allow
the VMM to allocate address ranges.

We are going to use a reserved memory region for restricting the 32-bit
memory hole from expanding beyond the IOAPIC and TSS addresses.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

arch/src/x86_64/mod.rs

@@ -77,11 +77,11 @@ pub fn arch_memory_regions(size: GuestUsize) -> Vec<(GuestAddress, usize, Region
         ));
     }
 
-    // Add the 32 bits hole as a "reserved" region.
+    // Add the 32 bits hole as a sub region.
     regions.push((
         memory_gap_start,
         MEM_32BIT_GAP_SIZE as usize,
-        RegionType::Reserved,
+        RegionType::SubRegion,
     ));
 
     regions