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

Revert submission 3408863

Reason for revert: Likely culprit for b/391356639  - verifying through ABTD before revert submission. This is part of the standard investigation process, and does not mean your CL will be reverted.

Reverted changes: /q/submissionid:3408863

Change-Id: Id9436b9557e0fd4cbc94df9f22ba35132619aa91
diff --git a/tests/backcompat_test/src/main.rs b/tests/backcompat_test/src/main.rs
index a7fb074..4113881 100644
--- a/tests/backcompat_test/src/main.rs
+++ b/tests/backcompat_test/src/main.rs
@@ -117,9 +117,10 @@
         /* consoleIn */ None,
         None,
         Some(dump_dt),
+        None,
     )
     .context("Failed to create VM")?;
-    vm.start(None).context("Failed to start VM")?;
+    vm.start().context("Failed to start VM")?;
     info!("Started example VM.");
 
     // Wait for VM to finish