docs/system: add basic virtio-gpu documentation

This adds basic documentation for virtio-gpu.

Suggested-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Alyssa Ross <hi@alyssa.is>
Tested-by: Emmanouil Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Emmanouil Pitsidianakis <manos.pitsidianakis@linaro.org>

v2: - Incorporated suggestions by Akihiko Odaki
    - Listed the currently supported capset_names (Bernard)

v3: - Incorporated suggestions by Akihiko Odaki and Alyssa Ross

v4: - Incorporated suggestions by Akihiko Odaki

v5: - Removed pci suffix from examples
    - Verified that -device virtio-gpu-rutabaga works.  Strangely
      enough, I don't remember changing anything, and I remember
      it not working.  I did rebase to top of tree though.
    - Fixed meson examples in crosvm docs

v8: - Remove different links for "rutabaga_gfx" and
      "gfxstream-enabled rutabaga" (Akihiko)

v11: - Use GPL-2.0-or-later license (Phillippe)
2 files changed