kvm: vmx: add cpu into VMX preemption timer bug list

This patch adds Intel "Xeon CPU E3-1220 V2", with CPUID.01H.EAX=0x000306A8,
into the list of known broken CPUs which fail to support VMX preemption
timer. This bug was found while running the APIC timer test of
kvm-unit-test on this specific CPU, even though the errata info can't be
located in the public domain for this CPU.

Signed-off-by: Wei Huang <wei@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>