x86: Clean up APIC local timer handling.

x86: Clean up APIC local timer handling.

1. Writing TMICT=0 disables the timer. Use this fact to simplify and
improve reprogram_timer(). In particular, we always write TMICT, and
write zero when we do not need a timer interrupt.

2. In HPET broadcast timer handler, set TMICT=0 when we mask the APIC
local timer. May as well do this early, before entering deep sleep.

3. In HVM-guest APIC emulation, disable the emulated local timer when
the guest sets TMICT=0. Previously we would issue an immediate
one-shot interrupt.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>