Restrict Contributors to view only their own custom post types?

You need to use the action hook pre_get_posts.

See the comments for details and modify the Custom Post Type(s) to your own:

add_action( 'pre_get_posts', 'filter_cpt_listing_by_author_wpse_89233' );

function filter_cpt_listing_by_author_wpse_89233( $wp_query_obj ) 
{
    // Front end, do nothing
    if( !is_admin() )
        return;

    global $current_user, $pagenow;
    wp_get_current_user();

    // http://php.net/manual/en/function.is-a.php
    if( !is_a( $current_user, 'WP_User') )
        return;

    // Not the correct screen, bail out
    if( 'edit.php' != $pagenow )
        return;

    // Not the correct post type, bail out
    if( 'portfolio' != $wp_query_obj->query['post_type'] )
        return;
        
    // If the user is not administrator, filter the post listing
    if( !current_user_can( 'delete_plugins' ) )
        $wp_query_obj->set('author', $current_user->ID );
}

You’ll notice that the post count All|Published|Drafts needs to be corrected.
See the solution here.