[platforms/virtual] Add virtual output class
Summary:
This matches the DRM backend more closely and allows mid-test removal and
addition of virtual outputs with different properties in the future.
Test Plan: Before and after 93% tests passed.
Reviewers: KWin, graesslin
Reviewed By: KWin, graesslin
Subscribers: graesslin, kwin, KWin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D11351