[kioslave/remote] Fix porting bugs

Summary:
Commit 72e39f08 broke the remote ioslave from Dolphin Details view
mode, because of missing slashes or otherwise broken `QUrl`s.

BUG: 376131
FIXED-IN: 5.9.2

Test Plan: `remote:/` now works again from Dolphin Details view mode.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4480