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>

pci/src/bus.rs

@@ -125,19 +125,16 @@ impl PciBus {
     pub fn register_mapping(
         &self,
         dev: Arc<dyn BusDeviceSync>,
-        #[cfg(target_arch = "x86_64")] io_bus: &Bus,
+        io_bus: &Bus,
         mmio_bus: &Bus,
         bars: Vec<PciBarConfiguration>,
     ) -> Result<()> {
         for bar in bars {
             match bar.region_type() {
                 PciBarRegionType::IoRegion => {
-                    #[cfg(target_arch = "x86_64")]
                     io_bus
                         .insert(dev.clone(), bar.addr(), bar.size())
                         .map_err(PciRootError::PioInsert)?;
-                    #[cfg(not(target_arch = "x86_64"))]
-                    error!("I/O region is not supported");
                 }
                 PciBarRegionType::Memory32BitRegion | PciBarRegionType::Memory64BitRegion => {
                     mmio_bus