xen: sched_credit: when picking, make sure we get an idle one, if any

xen: sched_credit: when picking, make sure we get an idle one, if any

The pcpu picking algorithm treats two threads of a SMT core the same.
More specifically, if one is idle and the other one is busy, they both
will be assigned a weight of 1. Therefore, when picking begins, if the
first target pcpu is the busy thread (and if there are no other idle
pcpu than its sibling), that will never change.

This change fixes this by ensuring that, before entering the core of
the picking algorithm, the target pcpu is an idle one (if there is an
idle pcpu at all, of course).

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Juergen Gross <juergen.gross@ts.fujitsu.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Keir Fraser <keir@xen.org>