7 – Drupal conditional .htaccess rule for multisiting sitemap.xml

We have multiple sites hosted with Domain access module. I was planning to host sitemap.xml files like

web_root/zyx.com/sitemap.xml

web_root/abc.com/sitemap.xml

I am applied something like this

RewriteCond %{HTTP_HOST} ^xzy.com [NC]
RewriteRule ^sitemap.xml$ /xzy.com/sitemap.xml

But this rule is not universal and requires to duplicate this rules every time new domain added. How this rule can be improved to serve sitemaps from web_root/website_name/sitemap.xml?

any help appreciated.