acpi, acpi_piix, vt82c686: factor out PM_TMR logic

factor out PM_TMR logic. Later This will be used by ich9 acpi.
Also fixes the same bug in vt82c686.c that was fixed by the following
commits.

> commit 055479fe
> Author: aliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162>
> Date:   Wed Jan 21 16:31:20 2009 +0000
>
>     Always return latest pmsts instead of the old one (Xiantao Zhang)
>
>     It may lead to the issue when booting windows guests with acpi=1
>     if return the old pmsts.
>
>     Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
>     Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

Cc: Blue Swirl <blauwirbel@gmail.com>
Cc: Huacai Chen <zltjiangshi@gmail.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>