commit 9ed96e87 upstream.

Limit PIT timer frequency similarly to the limit applied by
LAPIC timer.
Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.2:
 - Adjust context
 - s/ps->period/pt->period/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>