Don't create LUNs with nonexistant backing files/zvols.

If ctl.conf is blindly created with paths from the config
database, and those paths don't actually exist, then the targets
won't work anyways, so may as well exclude them.

Additionally if you generate the config file from the database
and don't skip invalid paths,
then start CTL, then import the volume, you'll just get confusing
error messages when you restart CTL.