libvmclient: Take VmCallback in VmInstance::start()

Taking VmCallback from VmInstance::new() has following issues:
  - Hard to manage VmInstance and VmCallback together,
    becaues VmInstance is available after Callback is instantiated.
  - VmCallback is only called after being started.

Bug: 381195543
Test: T/H
Change-Id: I013dce6371156ed298a612582c392a54768c00d6
diff --git a/microfuchsia/microfuchsiad/src/instance_starter.rs b/microfuchsia/microfuchsiad/src/instance_starter.rs
index e3c4e8d..7c4f32d 100644
--- a/microfuchsia/microfuchsiad/src/instance_starter.rs
+++ b/microfuchsia/microfuchsiad/src/instance_starter.rs
@@ -96,7 +96,6 @@
             console_in,
             /* log= */ None,
             /* dump_dt= */ None,
-            None,
         )
         .context("Failed to create VM")?;
         if let Some(pty) = &pty {
@@ -105,7 +104,7 @@
                 .setHostConsoleName(&pty.follower_name)
                 .context("Setting host console name")?;
         }
-        vm_instance.start().context("Starting VM")?;
+        vm_instance.start(None).context("Starting VM")?;
 
         Ok(MicrofuchsiaInstance {
             _vm_instance: vm_instance,