xl: make error reporting of cpupool subcommands consistent

xl: make error reporting of cpupool subcommands consistent

with the rest of the file, where we return 1 on 0, rather
than using libxl error codes.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Juergen Gross <JGross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>