Ftp ioslave: Fix ProxyUrls parameter passing

Summary:
QVariant stored in mapConfig always contain QByteArray values.
So QVariant needs to be converted string before splitting to StringList

Relates to D25432

Reviewers: trufanov, Frameworks

Reviewed By: trufanov

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25438