How do I migrate this module? I just migrate my Drupal 6 website to Drupal 7 successfully, using migrate_d2d module. Is there an easy way to migrate Drupal 6 xml sitemap to Drupal 7 too?

Should I just install the XML sitemap module in Drupal 7 and it'll create its own?
Your D7 may not remain unchanged forever.  When it change, you will want its sitemap to change in concert with the site. In other words, you need something like XML Sitemap on your site.

You should therefore install the XML Sitemap module (as you suggest yourself).  The module actually has an upgrade path if you used the D6 version of the module to generate the D6 sitemap.

If your D6 sitemap was generated by some other means, or if you don't care about preserving the D6 settings for the sitemap, just install at as a fresh module on D7, and it will create a new sitemap.