variable timer intervals

variable timer intervals

This patch introduces dynamic timer intervals: we slow down the refresh
rate when there in no much activity but we get back to a fast refresh
rate when the activity resume.

Please note that qemu_timer_expired is not an inline function any more
because I needed to call it from vnc.c however I don't think this change
should have any serious consequence.

Signed-off-by: Stefano Stabellini <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>
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