modules: fix reference to undefined variable in filemap.py

Looks like an omission. You'd run into this if you had multiple
mappings for the same file on the guest.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>