KVM: x86: use kvm_get_gdt() and kvm_read_ldt()

Use kvm_get_gdt() and kvm_read_ldt() to reduce inline assembly code.

Cc: Avi Kivity <avi@redhat.com>
Cc: kvm@vger.kernel.org
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>