Fix a case where spci_vm_id_t should be used.

Change-Id: I76dcf01cbafe1fd995e56515b9c4d856bd0fa215
diff --git a/src/api.c b/src/api.c
index 59a346d..698e788 100644
--- a/src/api.c
+++ b/src/api.c
@@ -208,7 +208,7 @@
 /**
  * Returns the ID of the VM.
  */
-int64_t api_vm_get_id(const struct vcpu *current)
+spci_vm_id_t api_vm_get_id(const struct vcpu *current)
 {
 	return current->vm->id;
 }