As per here: https://community.theforeman.org/t/issue-creating-kvms-with-libvirtd/11429/4

It looks as though Foreman (or at least the underlying fog-libvirt) is expecting the hypervisor to always support virDomainDefineXML which obviously isn’t the case.

For reference this Foreman instance (19) is running on RHEL 7.4 and the hypervisor is an up-to-date RHEL 6.10.

I’ve tested Foreman 11 on a different RHEL7 instance which was able to successfully create VMs on RHEL6 hypervisors. So at some point between Foreman 11 and 19 the libvirt-fog Gem(?) changed to a version that broke compatibility, which now means that Foreman won’t work with CentOS / RHEL 6 hypervisors.