savevm: Fix -loadvm to report errors to stderr, not the monitor

A monitor may not even exist.

Change load_vmstate() to use qemu_error() instead of monitor_printf().
Parameter mon is now unused, remove it.
diff --git a/vl.c b/vl.c
index fd89e50..0030d54 100644
--- a/vl.c
+++ b/vl.c
@@ -6072,7 +6072,7 @@
 
     qemu_system_reset();
     if (loadvm) {
-        if (load_vmstate(cur_mon, loadvm) < 0) {
+        if (load_vmstate(loadvm) < 0) {
             autostart = 0;
         }
     }