enable USE_PLATFORM_CLOCK bit in FADT flags

enable USE_PLATFORM_CLOCK bit in FADT flags

Enable bit 15 (USE_PLATFORM_CLOCK) of FADT flags field so that
older Windows guests do not make use of the TSC for timestamping.

From the ACPI manual:

"A value of one indicates that OSPM should use a platform provided timer
to drive any monotonically non-decreasing counters, such as OSPM
performance counter services. Which particular platform timer will be
used is OSPM specific, however, it is recommended that the timer used is
based on the following algorithm: If the HPET is exposed to OSPM, OSPM
should use the HPET. Otherwise, OSPM will use the ACPI power management
timer. A value of one indicates that the platform is known to have a
correctly implemented ACPI power management timer."

Problems using the TSC include:

1) Migration to a host with different frequency.
2) Unsynchronized TSCs in SMP hosts (KVM does not guarantee
synchronization).
3) Processors which do not increment TSC on low power states.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>