KVM: x86: Prepare kvm_hv_flush_tlb() to handle L2's GPAs

To handle L2 TLB flush requests, KVM needs to translate the specified
L2 GPA to L1 GPA to read hypercall arguments from there.

No functional change as KVM doesn't handle VMCALL/VMMCALL from L2 yet.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20221101145426.251680-14-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>