Add a virtio-gpu 2D driver

Add a driver to connect vt to the VirtIO GPU device in 2D mode. This
provides a output on the display when a qemu virtio gpu device is
added, e.g. with -device virtio-gpu-pci.

Tested on qemu using UTM, and a Hetzner arm64 VM instance.

Reviewed by: bryanv (earlier version)
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D40094