minigbm: Add virtio_gpu driver.

minigbm: Add virtio_gpu driver.

Add a minigbm driver to support the virtio_gpu DRM driver as used by
QEMU virtual machine.

BUG=chromium:591626
TEST=null_platform_test runs on emulated ARM64 virtual machine using
virtio_gpu driver and VIRTIO MMIO transport.

Change-Id: I1bbb060bb882d732b17435a2422cd7fb2c77a5f0
Reviewed-on: https://chromium-review.googlesource.com/332286
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>