Don't scan mounted disk image.

In 10.11 at least, a mounted Time Capsule volume is of type " Disk Image" when it used to be "Apple Disk Image". Scanning of such volume can take an extremely long time, which would cause hang when quitting as the MediaManager is waiting on the scan to complete before quitting

mythtv/libs/libmyth/mediamonitor-darwin.cpp

@@ -306,9 +306,9 @@ void diskAppearedCallback(DADiskRef disk, void *context)
         return;
     }
 
-    model     = getModel(details);
+    model = getModel(details);
 
-    if (model.contains("Apple Disk Image"))
+    if (model.contains("Disk Image"))
     {
         LOG(VB_MEDIA, LOG_INFO, msg + QString("DMG %1 mounted, ignoring")
             .arg(BSDname));