qemu: managedsave: Don't spam logs with warnings about corrupted image
    
    Even successful start of a VM from a managed save image would spam the
    logs with the following message:
    
    Unable to restore from managed state [path]. Maybe the file is
    corrupted?
    
    Re-arrange the logic to output the warning only when the image is
    corrupted.
    
    The flaw was introduced in commit cfc28c66.