migration: add comment for TYPE_MIGRATE

It'll be strange that the migration object inherits TYPE_DEVICE. Add
some explanations to it.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1498634144-26508-1-git-send-email-peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>