xen: Some code motion to avoid having to do forward-declaration

xen: Some code motion to avoid having to do forward-declaration

For sched_credit2, move the vcpu insert / remove / free functions near the domain
insert / remove / alloc / free functions (and after cpu_pick).

For sched_rt, move rt_cpu_pick() further up.

This is pure code motion; no functional change.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Meng Xu <mengxu@cis.upenn.edu>​
Acked-by: Dario Faggioli <dario.faggioli@citrix.com>