acpi: accurate overflow check

Compare clock in ns, because acpi_pm_tmr_update uses rounded
to ns value instead of ticks.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
[This lets Windows boot in icount mode. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
