s/KVM_X86_OP_RET0/KVM_X86_OP_OPTIONAL_RET0
And maybe "NULL func" instead of "NULL value", since some members of kvm_x86_ops
hold a value, not a func.
struct kvm_x86_ops will be changed to __static_call_return0.This implies kvm_x86_ops itself is changed, which is incorrect. "will be patched
to __static_call_return0() when updating static calls" or so.