KVM: Introduce kvm_memory_slot::arch and move lpage_info into it

Some members of kvm_memory_slot are not used by every architecture.

This patch is the first step to make this difference clear by
introducing kvm_memory_slot::arch;  lpage_info is moved into it.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 37e7f10..ff053ca 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -689,7 +689,7 @@
 	unsigned long idx;
 
 	idx = gfn_to_index(gfn, slot->base_gfn, level);
-	return &slot->lpage_info[level - 2][idx];
+	return &slot->arch.lpage_info[level - 2][idx];
 }
 
 static void account_shadowed(struct kvm *kvm, gfn_t gfn)