SPCI: Introduce SPCI_MSG_SEND.

Morph the vmapi service HF_MAILBOX_SEND onto SPCI_MSG_SEND.
The current SPCI_MSG_SEND only allows for implementation defined
messages to be exchanged between VMs.

The new SPCI service returns SPCI_SUCCESS if message is delivered or one
of the following error codes:
        SPCI_INVALID_PARAMETER: one of the parameters in the header does
        not conform;
        SPCI_BUSY: the mailbox was full or the target VM does not exist.

Adapted the tests to this new service.
Added tests specific to spci_message_init and spci_msg_send.

Change-Id: I55adfe68502ddc7bf864432f3e567b6cfe785f92
diff --git a/test/vmapi/primary_with_secondaries/inc/util.h b/test/vmapi/primary_with_secondaries/inc/util.h
index b127fae..eca641f 100644
--- a/test/vmapi/primary_with_secondaries/inc/util.h
+++ b/test/vmapi/primary_with_secondaries/inc/util.h
@@ -16,9 +16,11 @@
 
 #pragma once
 
+#include "vmapi/hf/spci.h"
+
 struct mailbox_buffers {
-	void *send;
-	void *recv;
+	struct spci_message *send;
+	struct spci_message *recv;
 };
 
 struct mailbox_buffers set_up_mailbox(void);