virtio-devices: Add virtio-balloon device

Add virtio-balloon device.
It only supports the basic functions of the virtio-balloon.

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

virtio-devices/src/lib.rs

@@ -29,6 +29,7 @@ use std::io;
 
 #[macro_use]
 mod device;
+pub mod balloon;
 pub mod block;
 mod console;
 mod iommu;
@@ -41,6 +42,7 @@ pub mod transport;
 pub mod vhost_user;
 pub mod vsock;
 
+pub use self::balloon::*;
 pub use self::block::*;
 pub use self::console::*;
 pub use self::device::*;