Make sure last enum values don't change when adding new ones before them

[vpodzime]

For example in #350, new enum values are added. Which means BD_NVDIMM_NAMESPACE_MODE_UNKNOWN, which should logically be last, now is a different integer value which is an ABI change. We should avoid this in the future by:

a) assign the enum values like the above one some high integer that won't change or b) assign all enum values explicit integer values using high values for the values that should stay last.