Revert^2 "libvmclient: Take VmCallback in VmInstance::start()"

3da8a161e1e5060b763f76686e63ccb622e76d31

Bug: 381195543
Test: T/H
Change-Id: I0edf795f9c10d444bc59b8ae57bf31671c03dae9
diff --git a/tests/backcompat_test/src/main.rs b/tests/backcompat_test/src/main.rs
index aa69eec..32830bc 100644
--- a/tests/backcompat_test/src/main.rs
+++ b/tests/backcompat_test/src/main.rs
@@ -117,10 +117,9 @@
         /* consoleIn */ None,
         None,
         Some(dump_dt),
-        None,
     )
     .context("Failed to create VM")?;
-    vm.start().context("Failed to start VM")?;
+    vm.start(None).context("Failed to start VM")?;
     info!("Started example VM.");
 
     // Wait for VM to finish