credit2: track residual from divisions done during accounting

credit2: track residual from divisions done during accounting

This should help with under-accounting of vCPU-s running for extremly
short periods of time, but becoming runnable again at a high frequency.

Don't bother subtracting the residual from the runtime, as it can only ever
add up to one nanosecond, and will end up being debited during the next
reset interval anyway.

Original-patch-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>