vmm: Switch RawFileDisk to DiskBackend::Next

Update device_manager.rs to construct DiskBackend::Next
instead of DiskBackend::Legacy for the io_uring raw backend.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

vmm/src/device_manager.rs

@@ -2756,9 +2756,7 @@ impl DeviceManager {
                         unreachable!("Checked in if statement above");
                         #[cfg(feature = "io_uring")]
                         {
-                            DiskBackend::Legacy(
-                                Box::new(RawFileDisk::new(file)) as Box<dyn DiskFile>
-                            )
+                            DiskBackend::Next(Box::new(RawFileDisk::new(file)))
                         }
                     } else if !disk_cfg.disable_aio && self.aio_is_supported() {
                         info!("Using asynchronous RAW disk file (aio)");