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>