sched: add some diagnostic info in the run queue keyhandler

sched: add some diagnostic info in the run queue keyhandler

When dumping the run queue information add some more data regarding
current and (if known) previous vcpu for each physical cpu.

With core scheduling activated the printed data will be e.g.:

(XEN) CPUs info:
(XEN) CPU[00] current=d[IDLE]v0, curr=d[IDLE]v0, prev=NULL
(XEN) CPU[01] current=d[IDLE]v1
(XEN) CPU[02] current=d[IDLE]v2, curr=d[IDLE]v2, prev=NULL
(XEN) CPU[03] current=d[IDLE]v3

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>