Logical x2APIC stops working if we rewrite it with zeros.
The best references are SDM April 2015: 10.12.10.1 Logical Destination
Mode in x2APIC Mode

  [...], the LDR are initialized by hardware based on the value of
  x2APIC ID upon x2APIC state transitions.

and SDM April 2015: 10.12.10.2 Deriving Logical x2APIC ID from the Local
x2APIC ID

  The LDR initialization occurs whenever the x2APIC mode is enabled

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>