Add nonempty flag

[Jason-ZW]

Add nonempty flag to fix share-folder bug.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "add-nonempty-flag" git@github.com:Jason-ZW/machine.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[Jason-ZW]

#4481 @dgageot Sorry for reply late. I rebase to master failed, so recreate a PR.