This is actually a KVM/qemu feature. "Xen" part was autofilled.
qemu 3.0 commit 0da0fb062 x86/cpu: Enable CLDEMOTE(Demote Cache Line) cpu feature
linux/kvm commit: 0ea3286e2d KVM: x86: Expose CLDEMOTE CPU feature to guest VM
This is actually a KVM/qemu feature. "Xen" part was autofilled.
qemu 3.0 commit 0da0fb062 x86/cpu: Enable CLDEMOTE(Demote Cache Line) cpu feature
linux/kvm commit: 0ea3286e2d KVM: x86: Expose CLDEMOTE CPU feature to guest VM