KVM: introduce id_to_memslot function

Introduce id_to_memslot to get memslot by slot id

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index ba24022..8f19d91 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2711,11 +2711,13 @@
 {
 	if (!kvm->arch.tss_addr) {
 		struct kvm_memslots *slots;
+		struct kvm_memory_slot *slot;
 		gfn_t base_gfn;
 
 		slots = kvm_memslots(kvm);
-		base_gfn = slots->memslots[0].base_gfn +
-				 kvm->memslots->memslots[0].npages - 3;
+		slot = id_to_memslot(slots, 0);
+		base_gfn = slot->base_gfn + slot->npages - 3;
+
 		return base_gfn << PAGE_SHIFT;
 	}
 	return kvm->arch.tss_addr;