Fix VirtualDesktopManager::createVirtualDesktop

Summary:
Currently, there are several issues with
VirtualDesktopManager::createVirtualDesktop:

(a) The method expects the number parameter to be in range [1, count + 1],
    but we pass [0, count];
(b) It doesn't correctly update X11 desktop numbers.

This change tries to address all previously mentioned issues.

BUG: 403312
FIXED-IN: 5.15.0

Test Plan: No longer able to reproduce bug 403312.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, hein, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18328