xenpm: assorted adjustments

xenpm: assorted adjustments

- use consistent error values (stop mixing of [positive] errno values
  with literal -E... ones)
- properly format output
- don't use leading zeros in decimal output
- move printing of average frequency into P-state conditional (rather
  than a C-state one)
- don't print some C-state related info when CPU idle management is
  disabled in the hypervisor
- use calloc() for array allocations that also got cleared via memset()
  so far

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>