[platforms/virtual] Store virtual outputs per reference

Summary:
Since VirtualOutput is a QObject, store it per reference and not per value.

Also fix some compilation warnings.

Test Plan: Before and after 92% tests passed.

Reviewers: #kwin, davidedmundson

Reviewed By: davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D11581