tools/virtio: fix build after 4.2 changes

more stubs, mostly

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio: separate headers more.

This makes them a bit more like the kernel headers, so we can include more
real kernel headers in our tests.

In addition this means that we don't break tools/virtio with the next
patch.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>