vga: optimize ppm_save() divisions ppm_save() spends upwards of 50% of its time doing divisions. Replace them with shifts. Reviewed-by: Alon Levy <alevy@redhat.com> Signed-off-by: Avi Kivity <avi@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>