hpet: Coding style cleanups and some refactorings

This moves the private HPET structures into the C module, simplifies
some helper functions and fixes most coding style issues (biggest chunk
was improper switch-case indention). No functional changes.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
