vmm: Add valid FDs for TAP devices to 'VmConfig::preserved_fds'

In this way, valid FDs for TAP devices will be closed when the holding
VmConfig instance is destroyed.

Signed-off-by: Bo Chen <chen.bo@intel.com>

vmm/src/device_manager.rs

@@ -2317,8 +2317,7 @@ impl DeviceManager {
                     .map_err(DeviceManagerError::CreateVirtioNet)?,
                 ))
             } else if let Some(fds) = &net_cfg.fds {
-                Arc::new(Mutex::new(
+                let net = virtio_devices::Net::from_tap_fds(
-                    virtio_devices::Net::from_tap_fds(
                     id.clone(),
                     fds,
                     Some(net_cfg.mac),
@@ -2332,8 +2331,14 @@ impl DeviceManager {
                         .map_err(DeviceManagerError::EventFd)?,
                     state,
                 )
-                    .map_err(DeviceManagerError::CreateVirtioNet)?,
+                .map_err(DeviceManagerError::CreateVirtioNet)?;
-                ))
+
+                // SAFETY: 'fds' are valid because TAP devices are created successfully
+                unsafe {
+                    self.config.lock().unwrap().add_preserved_fds(fds.clone());
+                }
+
+                Arc::new(Mutex::new(net))
             } else {
                 Arc::new(Mutex::new(
                     virtio_devices::Net::new(