Xen does not support changing this part of CPUID out of the box. Whatever you set in libvirt, Xen will always announce itself in CPUID 0x40xxxxxx leafs. Some people attempt patch Xen for that: Spoofing CPUID Results in the Linux-Stubdom Crashing · Issue #4980 · QubesOS/qubes-issues · GitHub, but as you can see, it isn’t fully successful.