Provide (some) VMs with multiple displays

We have a bunch of Qt APIs and framework code that deal with multiple displays in general and displays with different device-to-pixel-ratios and scale factors (DPRs). And we have a number of bugs reported against Qt's behavior when running on such systems.

We'd need some machines in our CI system that provide multiple virtual displays, each configured with a different DPR so that we can write and run respective unit tests. It's reasonably trivial to do that with locally run VMs (either through VirtualBox or VMware Fusion/Desktop), but we want unit test coverage and blocking CI.

A test run should include one configuration on each desktop platform to run on a system with multiple displays.