xen: credit1: don't rate limit context switches in case of yields

xen: credit1: don't rate limit context switches in case of yields

Rate limiting has been primarily introduced to avoid too
heavy context switch rate due to interrupts, and, in
general, asynchronous events.

If a vcpu "voluntarily" yields, we really should let it
give up the cpu for a while.

In fact, it may be that it is yielding because it's about
to start spinning, and there's few point in forcing a vcpu
to spin for (potentially) the entire rate-limiting period.

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