Bugfix: Fix metadata error on Windows for backups using VSS

Since restic 0.17.2, when creating a backup on Windows using `--use-fs-snapshot`,
restic would report an error like the following:

```
error: incomplete metadata for C:\: get EA failed while opening file handle for path \\?\GLOBALROOT\Device\HarddiskVolumeShadowCopyXX\, with: The process cannot access the file because it is being used by another process.
```

This has now been fixed by correctly handling paths that refer to volume
shadow copy snapshots.

https://github.com/restic/restic/issues/5107
https://github.com/restic/restic/pull/5110
https://github.com/restic/restic/pull/5112
