KVM: selftests: Fix off-by-one initialization of GDT limit

Fix an off-by-one bug in the initialization of the GDT limit, which as
defined in the SDM is inclusive, not exclusive.

Note, vcpu_init_descriptor_tables() gets the limit correct, it's only
vcpu_setup() that is broken, i.e. only tests that _don't_ invoke
vcpu_init_descriptor_tables() can have problems.  And the fact that KVM
effectively initializes the GDT twice will be cleaned up in the near
future.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
[sean: rewrite changelog]
Link: https://lore.kernel.org/r/20240314232637.2538648-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Ackerley Tng

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

@@ -523,7 +523,7 @@ static void kvm_setup_gdt(struct kvm_vm *vm, struct kvm_dtable *dt)
 		vm->arch.gdt = __vm_vaddr_alloc_page(vm, MEM_REGION_DATA);
 
 	dt->base = vm->arch.gdt;
-	dt->limit = getpagesize();
+	dt->limit = getpagesize() - 1;
 }
 
 static void kvm_setup_tss_64bit(struct kvm_vm *vm, struct kvm_segment *segp,