vm-allocator: Make port IO non optional

This is only for allocating the port IO address range.
If a platform does not have PIO devices at all, the address
range will simply be unused.
So, simplify the vm-allocator data structure by making both
MMIO and PIO mandatory.

Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>

vmm/src/vm.rs

@@ -1068,8 +1068,8 @@ impl<'a> Vm<'a> {
 
         // Let's allocate 64 GiB of addressable MMIO space, starting at 0.
         let mut allocator = SystemAllocator::new(
-            None,
-            None,
+            GuestAddress(0),
+            1 << 16 as GuestUsize,
             GuestAddress(0),
             1 << 36 as GuestUsize,
             X86_64_IRQ_BASE,