VSOCK: add send_byte()/recv_byte() test utilities

Test cases will want to transfer data.  This patch adds utility
functions to do this.

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>