Accurate accounting for credit scheduler

Rather than debit a full 10ms of credit on a scheduler tick
(probabilistic), debit credits accurately based on time stamps.

The main problem this is meant to address is an attack on the
scheduler that allows a rogue guest to avoid ever being debited
credits.  The basic idea is that the rogue process checks time (using
rdtsc) periodically, and yields after 9.5ms.  Using this technique, a
guest can "steal" 95% of the cpu.  This is particularly an issue in
cloud environments.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>

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