Comment 1 for bug 1702382

The suggestion from comment is to revert the -x removal when upstream has merged, but who knows how long that can take to get into stable kernels.

I think more likely we either:

1. Test aufs version and special case it to work around the bug.
2. Expect people to unmount volumes they don't want copied.

I think 2 is actually a more reasonable solution because the copy step appears to be executed not long after this step and it copies the image contents over regardless of filesystem and so it's possible that anything left mounted will be copied into an image that's too small. However i'm not familiar enough with the code to say that definitively.