minigbm: virtio_gpu: select dumb/virgl at runtime.

minigbm: virtio_gpu: select dumb/virgl at runtime.

For using same image on legacy-qemu and virgl enabled qemu. We need
to handle dumb and virgl code path at run time. Just merge them to
one driver.

BUG=b:77302150
TEST=manual - run novato image under qemu with gl=on/off.

Change-Id: Ia12140fba16a350fe13f4258d25333ecfe8352b3
Reviewed-on: https://chromium-review.googlesource.com/998631
Commit-Ready: Lepton Wu <lepton@chromium.org>
Tested-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>