docs: add README for runtime-rs hypervisor crate

add README for runtime-rs hypervisor crate

Fixes:#4634
Signed-off-by: Zhongtao Hu <zhongtaohu.tim@linux.alibaba.com>

src/runtime-rs/crates/hypervisor/README.md

@@ -0,0 +1,94 @@
+# Multi-vmm support for runtime-rs
+Some key points for supporting multi-vmm in rust runtime.
+## 1. Hypervisor Config
+
+The diagram below gives an overview for the hypervisor config
+
+![hypervisor config](../../docs/images/hypervisor-config.svg)
+
+VMM's config info will be loaded when initialize the runtime instance, there are some important functions need to be focused on. 
+### `VirtContainer::init()`
+
+This function initialize the runtime handler. It will register the plugins into the HYPERVISOR_PLUGINS. Different plugins are needed for different hypervisors. 
+```rust
+#[async_trait]
+impl RuntimeHandler for VirtContainer {
+    fn init() -> Result<()> {
+        // register
+        let dragonball_config = Arc::new(DragonballConfig::new());
+        register_hypervisor_plugin("dragonball", dragonball_config);
+        Ok(())
+    }
+}
+```
+
+[This is the plugin method for QEMU. Other VMM plugin methods haven't support currently.](../../../libs/kata-types/src/config/hypervisor/qemu.rs)
+QEMU plugin defines the methods to adjust and validate the hypervisor config file, those methods could be modified if it is needed.
+
+After that, when loading the TOML config, the plugins will be called to adjust and validate the config file.
+```rust
+async fn try_init(&mut self, spec: &oci::Spec) -> Result<()> {、
+    ...
+    let config = load_config(spec).context("load config")?;
+    ...
+}
+```
+
+### new_instance
+
+This function will create a runtime_instance which include the operations for container and sandbox.  At the same time, a hypervisor instance will be created.  QEMU instance will be created here as well, and set the hypervisor config file
+```rust
+async fn new_hypervisor(toml_config: &TomlConfig) -> Result<Arc<dyn Hypervisor>> {
+    let hypervisor_name = &toml_config.runtime.hypervisor_name;
+    let hypervisor_config = toml_config
+        .hypervisor
+        .get(hypervisor_name)
+        .ok_or_else(|| anyhow!("failed to get hypervisor for {}", &hypervisor_name))
+        .context("get hypervisor")?;
+
+    // TODO: support other hypervisor
+    match hypervisor_name.as_str() {
+        HYPERVISOR_DRAGONBALL => {
+            let mut hypervisor = Dragonball::new();
+            hypervisor
+                .set_hypervisor_config(hypervisor_config.clone())
+                .await;
+            Ok(Arc::new(hypervisor))
+        }
+        _ => Err(anyhow!("Unsupported hypervisor {}", &hypervisor_name)),
+    }
+}
+```
+
+## 2. Hypervisor Trait
+
+[To support multi-vmm, the hypervisor trait need to be implemented.](./src/lib.rs)
+```rust
+pub trait Hypervisor: Send + Sync {
+    // vm manager
+    async fn prepare_vm(&self, id: &str, netns: Option<String>) -> Result<()>;
+    async fn start_vm(&self, timeout: i32) -> Result<()>;
+    async fn stop_vm(&self) -> Result<()>;
+    async fn pause_vm(&self) -> Result<()>;
+    async fn save_vm(&self) -> Result<()>;
+    async fn resume_vm(&self) -> Result<()>;
+    
+    // device manager
+    async fn add_device(&self, device: device::Device) -> Result<()>;
+    async fn remove_device(&self, device: device::Device) -> Result<()>;
+    
+    // utils
+    async fn get_agent_socket(&self) -> Result<String>;
+    async fn disconnect(&self);
+    async fn hypervisor_config(&self) -> HypervisorConfig;
+    async fn get_thread_ids(&self) -> Result<VcpuThreadIds>;
+    async fn get_pids(&self) -> Result<Vec<u32>>;
+    async fn cleanup(&self) -> Result<()>;
+    async fn check(&self) -> Result<()>;
+    async fn get_jailer_root(&self) -> Result<String>;
+    async fn save_state(&self) -> Result<HypervisorState>;
+   }
+```
+In current design, VM will be started in the following steps.
+
+![vmm start](../../docs/images/vm-start.svg)