KVM: selftests: Move GDT, IDT, and TSS fields to x86's kvm_vm_arch

Now that kvm_vm_arch exists, move the GDT, IDT, and TSS fields to x86's
implementation, as the structures are firmly x86-only.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-4-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>