tools/virtio: add vring_test.

This is mainly to test the drivers/vhost/vringh.c code, but it also
uses the drivers/virtio/virtio_ring.c code for the guest side.

Usage for testing the basic implementation:

./vringh_test
# Test with indirect descriptors
./vringh_test --indirect
# Test with indirect descriptors and event indexex
./vringh_test --indirect --eventidx

You can run a parallel stress test by adding --parallel to any of the
above options.

eg ./vringh_test --parallel:
Using CPUS 0 and 3
Guest: notified 10107974, pinged 107970
Host: notified 108158, pinged 3172148

./vringh_test --indirect --eventidx --parallel:
Using CPUS 0 and 3
Guest: notified 156357, pinged 156251
Host: notified 156251, pinged 78179

Average of 50 times doing ./vringh_test --indirect --eventidx --parallel:
2.840000-3.040000(2.927292)user

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

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>