Introduce typedef for vCPU index.

Change-Id: I51194706edb86811487f2a22b3bda5405e605b5e
diff --git a/inc/vmapi/hf/abi.h b/inc/vmapi/hf/abi.h
index a625a8e..538a42e 100644
--- a/inc/vmapi/hf/abi.h
+++ b/inc/vmapi/hf/abi.h
@@ -90,7 +90,7 @@
 	union {
 		struct {
 			spci_vm_id_t vm_id;
-			uint16_t vcpu;
+			spci_vcpu_index_t vcpu;
 		} wake_up;
 		struct {
 			spci_vm_id_t vm_id;