moby: Windows: Failed to create volume over an existing directory

List the steps to reproduce the issue:

1. On Windows, create a directory C:\foo, and populate a file C:\foo\file
2. Attach a new volume at C:\foo
3. Observe the contents of C:\foo\file

Alternatively run CI test TestBuildVolumesRetainContents in docker_cli_builder_test.go against a Windows daemon (testRequires(c, DaemonIsLinux) needs to be commented out to run against a Windows daemon).

Describe the results you received: E:\bin\docker.exe: Error response from daemon: Cannot start container 7d3daab7ecfb827c3bbb61df21d249763606db688b3efa72c6c4bdfeeea61401: HCSShim::CreateComputeSystem- Win32 API call returned error r1=0x80070091 err=The directory is not empty. while attaching new volume in step 2.

Describe the results you expected: A new volume is attached at C:\foo, and the contents of C:\foo\file remain the same as initially added.

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@jhowardmsft @jstarks