kvm: LAPIC: Restore guard to prevent illegal APIC register access

In the Linux kernel, the following vulnerability has been resolved:

kvm: LAPIC: Restore guard to prevent illegal APIC register access

Per the SDM, "any access that touches bytes 4 through 15 of an APIC

register may cause undefined behavior and must not be executed."

Worse, such an access in kvm_lapic_reg_read can result in a leak of

kernel stack contents. Prior to commit 01402cf81051 ("kvm: LAPIC:

write down valid APIC registers"), such an access was explicitly

disallowed. Restore the guard that was removed in that commit.