Virtio GPUs are exposed over sysfs as having the virtio
bus type and a parent of the pci bus type.

In order to properly support these devices, a new bus
type needed to be added to avoid corner-cases while
pretending that the virtio devices are some other bus type.

Since the Virtio bus and device type doesn't expose
very much infomation, and none that would be unique
to a specific virtio device, bus and device info structs
are not added & populated for this bus type.

Signed-off-by: Robert Foss <robert.foss@collabora.com>