xen: sched: use default scheduler upon an invalid "sched="

xen: sched: use default scheduler upon an invalid "sched="

instead of just the first scheduler we find in the array.

In fact, right now, if someone makes a typo when passing
the "sched=" command line option to Xen, we (with all
schedulers configured in) pick ARINC653, which is most
likely not what one would expect.

Go for the default scheduler instead.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-By: Jonathan Creekmore <jonathan.creekmore@gmail.com>