virtio-balloon: add deflate_on_oom support

Sometimes we need balloon deflate automatically to give memory
back to guest, especially for some low priority guest processes
under memory pressure. Enable deflate_on_oom to support this.

Usage: --balloon "size=0,deflate_on_oom=on" \

Signed-off-by: Fei Li <lifei.shirley@bytedance.com>

vmm/src/device_manager.rs

@@ -2610,6 +2610,7 @@ impl DeviceManager {
                 virtio_devices::Balloon::new(
                     id.clone(),
                     balloon_config.size,
+                    balloon_config.deflate_on_oom,
                     self.seccomp_action.clone(),
                 )
                 .map_err(DeviceManagerError::CreateVirtioBalloon)?,