xenpm: make argument parsing and error handling more consistent

xenpm: make argument parsing and error handling more consistent

Specifically, what values are or aren't accepted as CPU identifier, and
how the values get interpreted should be consistent across sub-commands
(intended behavior now: non-negative values are okay, and along with
omitting the argument, specifying "all" will also be accepted).

For error handling, error messages should get consistently issued to
stderr, and the tool should now (hopefully) produce an exit code of
zero only in the (partial) success case (there may still be a small
number of questionable cases).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>