[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