[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