xen: credit1: simplify csched_runq_steal() a little bit.

xen: credit1: simplify csched_runq_steal() a little bit.

Since we're holding the lock on the pCPU from which we
are trying to steal, it can't have disappeared, so we
can drop the check for that (and convert it in an
ASSERT()).

And since we try to steal only from busy pCPUs, it's
unlikely for such pCPU to be idle, so we can:
 - tell the compiler this is actually unlikely,
 - bail early if the pCPU, unfortunately, turns out
   to really be idle.

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