vtllibrary: Update 'slot_type_str' into a wrapper function

Wrap slot_type_str[] into a function so it can be used by other consumers

Signed-off-by: Mark Harvey <markh794@gmail.com>
vtllibrary: Always log fatal error

If we can't open the config file for reading, always log the reason.

Signed-off-by: Mark Harvey <markh794@gmail.com>