Use a hash table to keep record of all loaded modules, and return early
if the requested module is already loaded.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1473043845-13197-3-git-send-email-famz@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>