Remove size field from MESSAGE return code.

The primary can read the message size from the SPCI header so doesn't
not need the value included in the return code.

Change-Id: I10ad11c502df9f39f45e12a81e00a99c4ebe1faf
diff --git a/src/abi_test.cc b/src/abi_test.cc
index f8cefa5..ed060e8 100644
--- a/src/abi_test.cc
+++ b/src/abi_test.cc
@@ -175,9 +175,8 @@
 {
 	struct hf_vcpu_run_return res = dirty_vcpu_run_return();
 	res.code = HF_VCPU_RUN_MESSAGE;
-	res.message.size = 0xdeadbeef;
 	res.message.vm_id = 0xf007;
-	EXPECT_THAT(hf_vcpu_run_return_encode(res), Eq(0xdeadbeeff0070005));
+	EXPECT_THAT(hf_vcpu_run_return_encode(res), Eq(0x0000000000f00705));
 }
 
 /**
@@ -188,8 +187,7 @@
 	struct hf_vcpu_run_return res =
 		hf_vcpu_run_return_decode(0x1123581314916205);
 	EXPECT_THAT(res.code, Eq(HF_VCPU_RUN_MESSAGE));
-	EXPECT_THAT(res.message.size, Eq(0x11235813));
-	EXPECT_THAT(res.message.vm_id, Eq(0x1491));
+	EXPECT_THAT(res.message.vm_id, Eq(0x9162));
 }
 
 /**
diff --git a/src/api.c b/src/api.c
index 16620a1..2454fc8 100644
--- a/src/api.c
+++ b/src/api.c
@@ -816,7 +816,6 @@
 
 	/* Messages for the primary VM are delivered directly. */
 	if (to->id == HF_PRIMARY_VM_ID) {
-		primary_ret.message.size = size,
 		to->mailbox.state = mailbox_state_read;
 		*next = api_switch_to_primary(current, primary_ret,
 					      vcpu_state_ready);