credit scheduler: fix credits overflow

credit scheduler: fix credits overflow

In changing credits-per-tick from 100 to 1000000, a possible overflow
was introduced in the accounting algorithm, when credit totals (which
can be in the millions) gets multiplied by a weight (typically 256):
th eresult can easily overflow a signed 32-bit variable.

Fix this by reverting to 100 credits per tick, and maintain long-term
fairness/correctness by tracking at the nanosecond level exactly how
much execution time has been accounted to each VCPU. We do this by
rounding execution time so far to nearest number of credits, but then
remember the VCPU's 'partial credit balance'.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>