Filesystem paths can be mapped from the host path to a guest path using
the format `<guest-path>::<host-path>`.
Previously `strtok` was used to find the `::` delimiter. Unfortunately
`strtok` processes each delimiter character individually. This meant
that the code was ~equivalent to `strtok(mapping_copy, ":")` which
breaks with Windows-style paths (E.g. `C:\my_path\`).
To fix this `strstr` is used to search for the exact delimiter.
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