KVM: Add kvm_vcpu_get_idx to get vcpu index in kvm->vcpus

There are occasional needs to use the index of vcpu in the kvm->vcpus
array to map something related to a VCPU.  For example, unlike the
vcpu->vcpu_id, the vcpu index is guaranteed to not be sparse across all
vcpus which is useful when allocating a memory area for each vcpu.
Signed-off-by: Christoffer Dall <cdall@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>

include/linux/kvm_host.h

@@ -490,6 +490,17 @@ static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id)
 	return NULL;
 }
 
+static inline int kvm_vcpu_get_idx(struct kvm_vcpu *vcpu)
+{
+	struct kvm_vcpu *tmp;
+	int idx;
+
+	kvm_for_each_vcpu(idx, tmp, vcpu->kvm)
+		if (tmp == vcpu)
+			return idx;
+	BUG();
+}
+
 #define kvm_for_each_memslot(memslot, slots)	\
 	for (memslot = &slots->memslots[0];	\
 	      memslot < slots->memslots + KVM_MEM_SLOTS_NUM && memslot->npages;\