Add macro definitions to report infrastructure for character array length.

Add macro definitions to report infrastructure for character array length.

Rather than hard code the size of the field, use a #define, so we can re-use.
The #define will be needed in a future patch when we extend the reporting
infrastructure to have 'get' and 'set' functions for each field, allowing
lvm2app functions which query any report field.  In order to provide a
generic lookup based on the field id, we will define a type containing this
field id, and thus, we will need to re-use the length of this string as
it's defined inside libdevmapper.h.