Fixes the default port for the vnc server.

The int overload of GetDefaultPerInstance was using the cmd line flag
instead of the default instance name.

Test: run locally
Change-Id: I27031160ca6eb01468c27735e33b90434376585e
diff --git a/host/libs/config/host_config.cpp b/host/libs/config/host_config.cpp
index 86c4be6..9acd663 100644
--- a/host/libs/config/host_config.cpp
+++ b/host/libs/config/host_config.cpp
@@ -50,7 +50,7 @@
 }
 
 int vsoc::GetPerInstanceDefault(int base) {
-  return base + FLAGS_instance - 1;
+  return base + GetDefaultInstance() - 1;
 }
 
 std::string vsoc::GetDefaultPerInstanceDir() {