config, device_manager: Add support for disabling io_uring for testing

Add config parameter to --disk called "_disable_io_uring" (the
underscore prefix indicating it is not for public consumpion.) Use this
option to disable io_uring if it would otherwise be used.

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

vmm/src/device_manager.rs

@@ -1642,7 +1642,7 @@ impl DeviceManager {
                 ImageType::Raw => {
                     // Use asynchronous backend relying on io_uring if the
                     // syscalls are supported.
-                    if block_io_uring_is_supported() {
+                    if block_io_uring_is_supported() && !disk_cfg.disable_io_uring {
                         let dev = Arc::new(Mutex::new(
                             virtio_devices::BlockIoUring::new(
                                 id.clone(),