[fedora-virt] Where do the machine definitions come from?

I'm upgrading a prehistoric fedora13 machine that hosted a gazillion virtual machines to centos 7. I find that the old virtual machine xml files make the new libvirt barf. I can get a good idea of how to fix all the old xml by installing a new file and comparing things, but I wonder where this magic comes from: <os> <type arch='x86_64' machine='pc-i440fx-rhel7.0.0'>hvm</type> <boot dev='hd'/> </os> Specifically, the "machine" definitions. Should that just always be the string above since my host is centos 7?