hpet: move out main counter read into a separate block

hpet: move out main counter read into a separate block

Follow up patche will switch main counter read to
lock-less mode. As preparation for that move relevant
branch into a separate top level block to make followup
patch cleaner/simplier by reducing contextual noise
when lock-less read is introduced.

no functional changes.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20250814160600.2327672-5-imammedo@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>