That code is also making use of a hardcoded 64-bit value as well.  Reading that as "kernel minimum address", that code is work-around for that fact that it is not detected or configured and libvmi is currently just using the 32-bit value.  Either:
1. OS specific constants can be handled withing the OS specific files, like START_KERNEL_map
2. We need to figure out how to set the value correctly, through configuration or detection, to make that __phys_addr() logic a little simpler.

One is obviously easier in the short term, 2 should probably be done eventually.  In regards to that specific piece of code, I would ask Zentific@ if they could define START_KERNEL_map32 and START_KERNEL_map64 in that code until we figure out 2.