Introduce spci_vm_count_t for count of VMs.

Change-Id: I4eae492136019c6a39b2d06b59793f021b4e07bb
diff --git a/inc/vmapi/hf/call.h b/inc/vmapi/hf/call.h
index 239244f..01be71f 100644
--- a/inc/vmapi/hf/call.h
+++ b/inc/vmapi/hf/call.h
@@ -56,7 +56,7 @@
 /**
  * Returns the number of secondary VMs.
  */
-static inline int64_t hf_vm_get_count(void)
+static inline spci_vm_count_t hf_vm_get_count(void)
 {
 	return hf_call(HF_VM_GET_COUNT, 0, 0, 0);
 }
diff --git a/inc/vmapi/hf/spci.h b/inc/vmapi/hf/spci.h
index 523c4ae..c387e62 100644
--- a/inc/vmapi/hf/spci.h
+++ b/inc/vmapi/hf/spci.h
@@ -60,8 +60,15 @@
 
 /* clang-format on */
 
+/** The ID of a VM. These are assigned sequentially. */
 typedef uint16_t spci_vm_id_t;
 
+/**
+ * A count of VMs. This has the same range as the VM IDs but we give it a
+ * different name to make the different semantics clear.
+ */
+typedef spci_vm_id_t spci_vm_count_t;
+
 /** SPCI common message header. */
 struct spci_message {
 	/*