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>

virtio-devices/src/device.rs

@@ -176,6 +176,28 @@ pub trait VirtioDevice: Send {
                 .unwrap();
         }
     }
+
+    /// Helper to allow common implementation of write_config
+    fn write_config_helper(&self, config: &mut [u8], offset: u64, data: &[u8]) {
+        let config_len = config.len() as u64;
+        let data_len = data.len() as u64;
+        if offset + data_len > config_len {
+            error!(
+                    "Out-of-bound access to configuration: config_len = {} offset = {:x} length = {} for {}",
+                    config_len,
+                    offset,
+                    data_len,
+                    self.device_type()
+                );
+            return;
+        }
+
+        if let Some(end) = offset.checked_add(config.len() as u64) {
+            let mut offset_config =
+                &mut config[offset as usize..std::cmp::min(end, config_len) as usize];
+            offset_config.write_all(data).unwrap();
+        }
+    }
 }
 
 /// Trait providing address translation the same way a physical DMA remapping