Implement a front-end driver of virtio-9p called vio9p

In conjunction with mount_9p, it enables a NetBSD system running as a VM guest
to mount an exported filesystem by the host via virtio-9p. It exports a 9p
end-point of virtio-9p via a character device file for mount_9p.

Reviewed by yamaguchi@

branches: 1.1.2; 1.1.4;
Reorg the virtio(4) config(9) code to be explicitly pci specific.

We'll later use this for a non-pci virtio(4) usecase.

ok martin

Reorg the virtio(4) config(9) code to be explicitly pci specific.

We'll later use this for a non-pci virtio(4) usecase.

ok martin