vsock/test: rework MSG_PEEK test for SOCK_STREAM

This new version makes test more complicated by adding empty read,
partial read and data comparisons between MSG_PEEK and normal reads.

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>