[XEN] Fix credit scheduler cap mechanism not to over park VCPUs

[XEN] Fix credit scheduler cap mechanism not to over park VCPUs
We used to park a capped VCPU when it had ran its fair share, even
if the fair share was below its cap. With this change, make sure
we only park once a VCPU has overrun its actual cap.
We could also try to make the capping mechanism more accurate by
parking and unparking at a finer granularity (currently done on
30ms boundaries) but that'll be for a different time.
Signed-off-by: Emmanuel Ackaouy <ack@xensource.com>