virtio-balloon: Add memory_actual_size to vm.info to show memory actual size

The virtio-balloon change the memory size is asynchronous.
VirtioBalloonConfig.actual of balloon device show current balloon size.

This commit add memory_actual_size to vm.info to show memory actual size.

Signed-off-by: Hui Zhu <teawater@antfin.com>

vmm/src/vm.rs

@@ -1493,6 +1493,11 @@ impl Vm {
 
         Ok(())
     }
+
+    /// Gets the actual size of the balloon.
+    pub fn get_balloon_actual(&self) -> u64 {
+        self.memory_manager.lock().unwrap().get_balloon_actual()
+    }
 }
 
 impl Pausable for Vm {