vmm: Do the downcast of GicDevice in a safer way for AArch64

Downcasting of GicDevice trait might fail. Therefore we try to
downcast the trait first and only if the downcasting succeeded we
can then use the object to call methods. Otherwise, do nothing and
log the failure.

Signed-off-by: Henry Wang <Henry.Wang@arm.com>

vmm/src/device_manager.rs

@@ -4147,14 +4147,19 @@ impl Pausable for DeviceManager {
                     .get_gic_device()
                     .unwrap(),
             );
-            gic_device
+            if let Some(gicv3_its) = gic_device
                 .lock()
                 .unwrap()
                 .as_any_concrete_mut()
                 .downcast_mut::<KvmGicV3Its>()
-                .unwrap()
-                .pause()?;
-        }
+            {
+                gicv3_its.pause()?;
+            } else {
+                return Err(MigratableError::Pause(anyhow!(
+                    "GicDevice downcast to KvmGicV3Its failed when pausing device manager!"
+                )));
+            };
+        };
 
         Ok(())
     }