virtio-devices: introduce ActivationContext for device activation

Signed-off-by: Peter Oskolkov <posk@google.com>
(cherry picked from commit f77c6ef78b)

vmm/src/device_manager.rs

@@ -4477,7 +4477,7 @@ impl DeviceManager {
     }
 
     pub fn activate_virtio_devices(&self) -> DeviceManagerResult<()> {
-        for mut activator in self.pending_activations.lock().unwrap().drain(..) {
+        for activator in self.pending_activations.lock().unwrap().drain(..) {
             activator
                 .activate()
                 .map_err(DeviceManagerError::VirtioActivate)?;