Evaluate xserver-xorg-video-modesetting for KMS support in QEMU and others

It would be good to support KMS for more video adapters in Tails, especially to allow migrating to GNOME Classic in Jessie (#7311 (closed)), and to make our upcoming custom Plymouth theme (#5948) work on a broader range of hardware. This would also generally improve the user story we provide to people who use Tails in virtual machines, e.g. with the QEMU (virtual) Cirrus video adapter.

Feature Branch: feature/kms-for-cirrus-etc

Parent Task: #7311 (closed)

Related issues

Related to #5948
Related to #7564 (closed)
Has duplicate #7565 (closed)

Original created by @intrigeri on 7394 (Redmine)

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