The version of Snow Leopard for PowerPC does not officially exist, it is a manual process that involves modifying a Developer Beta of Snow Leopard to boot on PowerPC. The steps for this have not been written out, as it involves a lot of prep work before being able to see that nice Snow Leopard desktop. Wait until this warning is no longer present.
This area describes how to use PowerPC and run Mac OS X via Virt-Manager with DarwinPPC.xml