cfg: fix config save atomicity (#760)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Fully flush `tmp` file and then use `MoveFileExW` instead of `CopyFileW`
which guarantees atomic operation on NTFS.

## Testing

src/spice2x/overlay/windows/card_manager.cpp

@@ -536,7 +536,7 @@ namespace overlay::windows {
     }
 
     void CardManager::config_save() {
-        log_info("cardmanager", "saving config");
+        log_misc("cardmanager", "saving config...");
 
         // create document
         Document doc;