vsock/test: MSG_PEEK test for SOCK_SEQPACKET

This adds MSG_PEEK test for SOCK_SEQPACKET. It works in the same way as
SOCK_STREAM test, except it also tests MSG_TRUNC flag.

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