Make the mediatype selection configurable.

Add an extra config option "device reserve by mediatype" which controls
if we try to reserve a certain non autochanger device by the mediatype
it has configured.

Some people configure different different devices with the same media
type. Although this will lead to all kind of problems lets make the new
device reserve by mediatype something you have to explictly enable as
probably very little people need this very specific reservation type.