Add new argument to hf_mailbox_send.

We don't do anything with it yet, but adding it in a separate patch
reduces the mix of functionality and refactor in the next patch.

Change-Id: I24cbd4d35c4cf9e4985a72d336a68c496c9cc2be
diff --git a/src/api.c b/src/api.c
index 74c45c7..004122f 100644
--- a/src/api.c
+++ b/src/api.c
@@ -405,9 +405,12 @@
 /**
  * Copies data from the sender's send buffer to the recipient's receive buffer
  * and notifies the recipient.
+ *
+ * If the recipient's receive buffer is busy, it can optionally register the
+ * caller to be notified when the recipient's receive buffer becomes available.
  */
-int64_t api_mailbox_send(uint32_t vm_id, size_t size, struct vcpu *current,
-			 struct vcpu **next)
+int64_t api_mailbox_send(uint32_t vm_id, size_t size, bool notify,
+			 struct vcpu *current, struct vcpu **next)
 {
 	struct vm *from = current->vm;
 	struct vm *to;
@@ -415,6 +418,8 @@
 	uint16_t vcpu;
 	int64_t ret;
 
+	(void)notify;
+
 	/* Limit the size of transfer. */
 	if (size > HF_MAILBOX_SIZE) {
 		return -1;