vhost_net: Add self test with tun device

vhost_net: Add self test with tun device

This creates a tun device and brings it up, and finds out the link-local
address that the kernel automatically assigns to it. It then sets up
vhost-net on the tun device, uses that to send a ping to the kernel's
assigned link-local addresss, and waits for a reply.

If everything is working correctly, it will get a response and manage to
understand it. If the virtio_net_hdr and other pieces are not working as
expected, then it fails (and times out after 1 second).

The test is repeated in various combinations of vhost-net feature flags,
tun vhdr length, PI enabled, and XDP/non-XDP code paths. Most of which
didn't work before the patch series that added this test, but do now.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>