Support cpupools in xl sched-credit

Support cpupools in xl sched-credit

Adds cpupool awareness to output of xl sched-credit. Output can now be
restricted to a specific cpupool. The domains are printed for each cpupool
seperately.

The loop over cpupools and domains is seperated from the main command
implementation to be able to support other schedulers as well.

Signed-off-by: juergen.gross@ts.fujitsu.com
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>