KVM: Per-vcpu statistics

Make the exit statistics per-vcpu instead of global.  This gives a 3.5%
boost when running one virtual machine per core on my two socket dual core
(4 cores total) machine.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/mmu.c b/drivers/kvm/mmu.c
index 8ccf84e..32c64f6 100644
--- a/drivers/kvm/mmu.c
+++ b/drivers/kvm/mmu.c
@@ -936,7 +936,7 @@
 
 static void kvm_mmu_flush_tlb(struct kvm_vcpu *vcpu)
 {
-	++kvm_stat.tlb_flush;
+	++vcpu->stat.tlb_flush;
 	kvm_arch_ops->tlb_flush(vcpu);
 }