vmm: add helper to measure successful operation duration

Add a small helper that returns both the successful result of an
operation and the time it took to complete.

Subsequent migration instrumentation uses this to keep timing code
compact and consistent.

On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

vmm/src/lib.rs

@@ -563,6 +563,17 @@ pub fn start_vmm_thread(
     })
 }
 
+/// Measures the time of the callback, in case it returns `Ok`.
+fn measure_ok<T, E, F>(f: F) -> result::Result<(T, Duration), E>
+where
+    F: FnOnce() -> result::Result<T, E>,
+{
+    let begin = Instant::now();
+    let value = f()?;
+    let duration = begin.elapsed();
+    Ok((value, duration))
+}
+
 #[derive(Clone, Deserialize, Serialize)]
 struct VmMigrationConfig {
     vm_config: Arc<Mutex<VmConfig>>,