KVM: selftests: Allocate x86's GDT during VM creation

Allocate the GDT during creation of non-barebones VMs instead of waiting
until the first vCPU is created, as the whole point of non-barebones VMs
is to be able to run vCPUs, i.e. the GDT is going to get allocated no
matter what.
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-13-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/lib/x86_64/processor.c

@@ -519,9 +519,6 @@ vm_paddr_t addr_arch_gva2gpa(struct kvm_vm *vm, vm_vaddr_t gva)
 
 static void kvm_setup_gdt(struct kvm_vm *vm, struct kvm_dtable *dt)
 {
-	if (!vm->arch.gdt)
-		vm->arch.gdt = __vm_vaddr_alloc_page(vm, MEM_REGION_DATA);
-
 	dt->base = vm->arch.gdt;
 	dt->limit = getpagesize() - 1;
 }
@@ -645,6 +642,7 @@ static void vm_init_descriptor_tables(struct kvm_vm *vm)
 	extern void *idt_handlers;
 	int i;
 
+	vm->arch.gdt = __vm_vaddr_alloc_page(vm, MEM_REGION_DATA);
 	vm->arch.idt = __vm_vaddr_alloc_page(vm, MEM_REGION_DATA);
 	vm->handlers = __vm_vaddr_alloc_page(vm, MEM_REGION_DATA);
 	/* Handlers have the same address in both address spaces.*/