vmm: support PCI I/O regions on all architectures

While non-Intel CPU architectures don't have a special concept of IO
address space, support for PCI I/O regions is still needed to be able
to handle PCI devices that use them.

With this change, I'm able to pass through an e1000e device from QEMU
to a cloud-hypervisor VM on aarch64 and use it in the cloud-hypervisor
guest.  Previously, it would hit the unimplemented!().

Signed-off-by: Alyssa Ross <hi@alyssa.is>

vmm/src/memory_manager.rs

@@ -1159,14 +1159,8 @@ impl MemoryManager {
 
         let allocator = Arc::new(Mutex::new(
             SystemAllocator::new(
-                #[cfg(target_arch = "x86_64")]
-                {
-                    GuestAddress(0)
-                },
-                #[cfg(target_arch = "x86_64")]
-                {
-                    1 << 16
-                },
+                GuestAddress(0),
+                1 << 16,
                 start_of_platform_device_area,
                 PLATFORM_DEVICE_AREA_SIZE,
                 #[cfg(target_arch = "x86_64")]