hpet: make main counter read lock-less

hpet: make main counter read lock-less

Make access to main HPET counter lock-less.

In unlikely event of an update in progress, readers will busy wait
untill update is finished.

As result micro benchmark of concurrent reading of HPET counter
with large number of vCPU shows over 80% better (less) latency.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20250814160600.2327672-6-imammedo@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>