vmm: Implement Clone and Drop for VmConfig

The custom 'clone' duplicates 'preserved_fds' so that the validation
logic can be safely carried out on the clone of the VmConfig.

The custom 'drop' ensures 'preserved_fds' are safely closed when the
holding VmConfig instance is destroyed.

Signed-off-by: Bo Chen <chen.bo@intel.com>

vmm/src/vm_config.rs

@@ -563,7 +563,7 @@ pub struct TpmConfig {
     pub socket: PathBuf,
 }
 
-#[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
+#[derive(Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct VmConfig {
     #[serde(default)]
     pub cpus: CpusConfig,