Introduce typedef for vCPU index.

Change-Id: I51194706edb86811487f2a22b3bda5405e605b5e
diff --git a/inc/vmapi/hf/spci.h b/inc/vmapi/hf/spci.h
index c387e62..25e8e83 100644
--- a/inc/vmapi/hf/spci.h
+++ b/inc/vmapi/hf/spci.h
@@ -68,6 +68,7 @@
  * different name to make the different semantics clear.
  */
 typedef spci_vm_id_t spci_vm_count_t;
+typedef uint16_t spci_vcpu_index_t;
 
 /** SPCI common message header. */
 struct spci_message {