vnc: add a way to get the update frequency for a given region

vnc: add a way to get the update frequency for a given region

This patch compute the update frequency (in Hz) for each 64x64 rects.
Any adaptive encoding can get this value using vnc_update_freq(), and
switch to a lossy encoding if the value is too high.

The frequency is pre-calculated every 500ms, based on the last 10
updates per 64x64 rect.

If a 64x64 rect was not updated in the last 2 second, then the frequency
became 0, and all the stored timestamp are reseted.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>