hvm pmtimer: correct pmtimer accuracy

hvm pmtimer: correct pmtimer accuracy

Several seconds of backward time drift per minute can be seen on a
RHEL6 HVM guest by switching the clocksource to 'acpi_pm' and then
running gettimeofday() in a loop. This is due to the accumulation
of small inaccuracies that are caused by shifting out the lower 32
bits when pmt_update_time() computes 'tmr_val'.

The patch makes sure that the lower 32 bits of the computed value
are not lost. They are saved in a new field 'not_accounted' in the
PMTState structure and are accounted the next time pmt_update_time()
is called.

From: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>