vm/vmm: properly shutdown output reading goroutine

The goroutine sends on bootOutputStop to notify about its completion,
but the main goroutine is not receiving from the chan on success
and since the chan in unbuffered, the output reading goroutine
hangs on the send forever.

vm/vmm/vmm.go

@@ -196,6 +196,7 @@ func (inst *instance) Boot() error {
 		return vmimpl.BootError{Title: err.Error(), Output: bootOutput}
 	}
 	bootOutputStop <- true
+	<-bootOutputStop
 	return nil
 }