vnc: Remove default_mon usage

These errors don't seem user initiated, so forcibly printing to the
monitor doesn't seem right. Just use error_report.

Cc: Anthony Liguori <aliguori@amazon.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

ui/vnc.c

@@ -996,7 +996,7 @@ static void audio_add(VncState *vs)
     struct audio_capture_ops ops;
 
     if (vs->audio_cap) {
-        monitor_printf(default_mon, "audio already running\n");
+        error_report("audio already running");
         return;
     }
 
@@ -1006,7 +1006,7 @@ static void audio_add(VncState *vs)
 
     vs->audio_cap = AUD_add_capture(&vs->as, &ops, vs);
     if (!vs->audio_cap) {
-        monitor_printf(default_mon, "Failed to add audio capture\n");
+        error_report("Failed to add audio capture");
     }
 }