test/vsock: vsock_perf utility

This adds utility to check vsock rx/tx performance.

Usage as sender:
./vsock_perf --sender <cid> --port <port> --bytes <bytes to send>
Usage as receiver:
./vsock_perf --port <port> --rcvlowat <SO_RCVLOWAT>

Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>