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/vm.rs

@@ -531,7 +531,6 @@ impl Vm {
         let stop_on_boot = false;
 
         let memory = memory_manager.lock().unwrap().guest_memory();
-        #[cfg(target_arch = "x86_64")]
         let io_bus = Arc::new(Bus::new());
         let mmio_bus = Arc::new(Bus::new());
 
@@ -622,7 +621,6 @@ impl Vm {
         let dynamic = true;
 
         let device_manager = DeviceManager::new(
-            #[cfg(target_arch = "x86_64")]
             io_bus,
             mmio_bus,
             vm.clone(),