xl: fix `xl cpupool-list' behavior in case no pool name is provided

xl: fix `xl cpupool-list' behavior in case no pool name is provided

since it errors out, asking for at least one argument, and does
not display any useful output, which is wrong (we want the list
and the info about all the existing cpupools).

IOW, the output is as follows:

 ~# xl cpupool-list -c
 'xl cpupool-list' requires at least 1 argument.
 ...

While it should be as follows:

 ~# xl cpupool-list -c
 Name               CPU list
 Pool-0             0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Juergen Gross <juergen.gross@ts.fujitsu.com>