vmm: Add virtio-balloon support

This commit adds new option balloon to memory config.
Set it to on will open the balloon function.

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

src/main.rs

@@ -502,6 +502,7 @@ mod unit_tests {
                     hotplug_size: None,
                     shared: false,
                     hugepages: false,
+                    balloon: false,
                 },
                 kernel: Some(KernelConfig {
                     path: PathBuf::from("/path/to/kernel"),