Allow use of xmlsitemap base url setting instead of actual subdomain in the event the domain has an alias

If a user has a custom url pointing to their subdomain, the xmlsitemap should return the custom url in the links--not the subdomain. Example:

Domain alias = custom.com -> sub.parent.com

xmlsitemap currently for page /xyz:
sub.parent.com/xyz

Should be able to return sitemap using the base url setting entered at admin/settings/xmlsitemap/settings so it returns the following xmlsitemap for page /xyz:
custom.com/xyz