print online pCPUs and free pCPUs when dumping

print online pCPUs and free pCPUs when dumping

e.g., with  `xl debug-key r', like this:

  (XEN) Online Cpus: 0-15
  (XEN) Free Cpus: 8-15

Also, for each cpupool, print the set of pCPUs it
contains, like this:

  (XEN) Cpupool 0:
  (XEN) Cpus: 0-7
  (XEN) Scheduler: SMP Credit Scheduler (credit)

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Juergen Gross <jgross@suse.com>
cpumask_weight() -> !cpumask_empty()

Signed-off-by: Jan Beulich <jbeulich@suse.com>