xen: credit2: soft-affinity awareness in gat_fallback_cpu()

xen: credit2: soft-affinity awareness in gat_fallback_cpu()

By, basically, moving all the logic of the function
inside the usual two steps (soft-affinity step and
hard-affinity step) loop.

While there, add two performance counters (in cpu_pick
and in get_fallback_cpu() itself), in order to be able
to tell how frequently it happens that we need to look
for a fallback cpu.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Signed-off-by: Justin T. Weaver <jtweaver@hawaii.edu>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>