VSOCK: add AF_VSOCK test cases

The vsock_test.c program runs a test suite of AF_VSOCK test cases.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>