vmm: device_manager: Allocate I/O port for ACPI shutdown device

The refactoring in ce1765c8af033bfa7d08a7bf27ac6b560f06d435 dropped the
code to allocate the I/O port.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/device_manager.rs

@@ -669,6 +669,13 @@ impl DeviceManager {
                 exit_evt, reset_evt,
             )));
 
+            address_manager
+                .allocator
+                .lock()
+                .unwrap()
+                .allocate_io_addresses(Some(GuestAddress(0x3c0)), 0x8, None)
+                .ok_or(DeviceManagerError::AllocateIOPort)?;
+
             address_manager
                 .io_bus
                 .insert(acpi_device.clone(), 0x3c0, 0x4)