selftests: KVM: Replace get_{gdt,idt}_base() by get_{gdt,idt}()

get_gdt_base() and get_idt_base() only return the base address
of the descriptor tables. Soon we will need to get the size as well.
Change the prototype of those functions so that they return
the whole desc_ptr struct instead of the address field.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Wei Huang <wei.huang2@amd.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>