Implement SPCI_ID_GET, and a convenience wrapper for clients.

Change-Id: Ic85256bc71002380ff16ebc2af8206af00f66654
diff --git a/src/api.c b/src/api.c
index d11b305..5261688 100644
--- a/src/api.c
+++ b/src/api.c
@@ -210,9 +210,10 @@
 /**
  * Returns the ID of the VM.
  */
-spci_vm_id_t api_vm_get_id(const struct vcpu *current)
+struct spci_value api_spci_id_get(const struct vcpu *current)
 {
-	return current->vm->id;
+	return (struct spci_value){.func = SPCI_SUCCESS_32,
+				   .arg2 = current->vm->id};
 }
 
 /**