KVM: Keep an upper bound of initialized vcpus

That way, we don't need to loop for KVM_MAX_VCPUS for a single vcpu
vm.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/kvm.h

@@ -379,6 +379,7 @@ struct kvm {
 	struct list_head active_mmu_pages;
 	int n_free_mmu_pages;
 	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
+	int nvcpus;
 	struct kvm_vcpu vcpus[KVM_MAX_VCPUS];
 	int memory_config_version;
 	int busy;

drivers/kvm/kvm_main.c

@@ -2391,6 +2391,11 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, int n)
 	if (r < 0)
 		goto out_free_vcpus;
 
+	spin_lock(&kvm_lock);
+	if (n >= kvm->nvcpus)
+		kvm->nvcpus = n + 1;
+	spin_unlock(&kvm_lock);
+
 	return r;
 
 out_free_vcpus: