Return vm_lock directly rather than through an argument.

Change-Id: I3acc96b32574c3a443210434cb17fafff46e430e
diff --git a/src/vm.c b/src/vm.c
index f2bbe78..e136e84 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -87,10 +87,15 @@
 /**
  * Locks the given VM and updates `locked` to hold the newly locked vm.
  */
-void vm_lock(struct vm *vm, struct vm_locked *locked)
+struct vm_locked vm_lock(struct vm *vm)
 {
+	struct vm_locked locked = {
+		.vm = vm,
+	};
+
 	sl_lock(&vm->lock);
-	locked->vm = vm;
+
+	return locked;
 }
 
 /**