[v2] xl: Don't require a config file for cpupools

[v2] xl: Don't require a config file for cpupools

Since the key information can be fairly simply put on the command-line,
there's no need to require an actual config file.

Also improve the help to cross-reference the xlcpupool.cfg manpage.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>