Monitor: Convert do_cont() to cmd_new_ret()

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/monitor.c b/monitor.c
index c1e0af8..cede368 100644
--- a/monitor.c
+++ b/monitor.c
@@ -1148,14 +1148,18 @@
 /**
  * do_cont(): Resume emulation.
  */
-static void do_cont(Monitor *mon, const QDict *qdict, QObject **ret_data)
+static int do_cont(Monitor *mon, const QDict *qdict, QObject **ret_data)
 {
     struct bdrv_iterate_context context = { mon, 0 };
 
     bdrv_iterate(encrypted_bdrv_it, &context);
     /* only resume the vm if all keys are set and valid */
-    if (!context.err)
+    if (!context.err) {
         vm_start();
+        return 0;
+    } else {
+        return -1;
+    }
 }
 
 static void bdrv_key_cb(void *opaque, int err)