ratelimit: Implement rate limit for credit2 scheduler

ratelimit: Implement rate limit for credit2 scheduler

Rate limit assures that a vcpu will execute for a minimum amount of
time before being put at the back of a queue or being preempted by
higher priority thread.

It introduces context-switch rate-limiting. The patch enables the VM
to batch its work and prevents the system from spending most of its
time in context switches because of a VM that is waking/sleeping at
high rate.

ratelimit can be disabled by setting it to 0.

Signed-off-by: Anshul Makkar <anshul.makkar@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>