xen: credit2: implement yield()

xen: credit2: implement yield()

When a vcpu explicitly yields it is usually giving
us an advice of "let someone else run and come back
to me in a bit."

Credit2 isn't, so far, doing anything when a vcpu
yields, which means an yield is basically a NOP (well,
actually, it's pure overhead, as it causes the scheduler
kick in, but the result is --at least 99% of the time--
that the very same vcpu that yielded continues to run).

With this patch, when a vcpu yields, we go and try
picking the next vcpu on the runqueue that can run on
the pcpu where the yielding vcpu is running. Of course,
if we don't find any other vcpu that wants and can run
there, the yielding vcpu will continue.

Also, add an yield performance counter, and fix the
style of a couple of comments.

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