x86/HVM: assorted RTC emulation adjustments

x86/HVM: assorted RTC emulation adjustments

- only call check_update_timer() on REG_B writes when SET changes
- only call alarm_timer_update() on REG_B writes when relevant bits
  change
- instead properly handle AF and PF when the guest is not also setting
  AIE/PIE respectively (for UF this was already the case, only a
  comment was slightly inaccurate), including calling the respective
  update functions upon REG_C reads

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Olaf Hering <olaf@aepfle.de>