We should  use the empty values above before adding more.
The struct lttng_version should not be in lttng.h but rather in mi-lttng.h.

lttng.h is the *public* ABI thus data structure being used for the public API
 used by lttng_* function calls.

Thus padding won't be necessary. Also, uint32_t is preferred to int.

Refactor the name to mi_lttng_version according to the mi namespace.
Create a function to print the mi (print_mi()) since we *may* support  many
output type.

This allow to handle the mi writer destruction in one specific call.
Also, cleaner in the long run.
We should ignore the case to allow anything that look like "xml".
Remove the default return values in mi_lttng_.
Don't need the HIDDEN attribute since they are assigned in the C file.