KVM: x86: check for nEPT/nNPT in slow flush hypercalls

In the Linux kernel, the following vulnerability has been resolved: KVM: x86: check for nEPT/nNPT in slow flush hypercalls Checking is_guest_mode(vcpu) is incorrect, because translate_nested_gpa() is only valid if an L2 guest is running *with nested EPT/NPT enabled*. Instead use the same condition as translate_nested_gpa() itself.