mc146818rtc: fix timer interrupt reinjection again

Commit 369b4135 broke timer interrupt
reinjection when there is no period change by the guest.  In that
case, old_period is 0, which ends up zeroing irq_coalesced (counter of
reinjected interrupts).

The consequence is Windows 7 is unable to synchronize time via NTP.
Easily reproducible by playing a fullscreen video with cirrus and VNC.

Fix by passing s->period when periodic_timer_update is called due to
expiration of the timer.  With this change, old_period == 0 only
means that the periodic timer was off.

Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Co-developed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>