i8253: Disable PIT timer 0 when not in use

i8253: Disable PIT timer 0 when not in use

Leaving the PIT interrupt running can cause noticeable steal time for
virtual guests. The VMM generally has a timer which toggles the IRQ input
to the PIC and I/O APIC, which takes CPU time away from the guest. Even
on real hardware, running the counter may use power needlessly (albeit
not much).

Make sure it's turned off if it isn't going to be used.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>