xl: fix incorrect display of illegal option character

xl: fix incorrect display of illegal option character

according to the getopt(3) manpage (and to my testing) getopt returns
'?' if an unknown option character is found and stores the insulting
character in optopt.
This patch fixes the broken output in such a situation:

root@dosorca:/data/images# xl vcpu-list -j
option `?' not supported.
Name                  ID  VCPU   CPU State   Time(s) CPU Affinity
Domain-0               0     0    0   -b-     193.1  any cpu

turns into:

root@dosorca:/data/images# xl vcpu-list -j
option `j' not supported.
Name                  ID  VCPU   CPU State   Time(s) CPU Affinity
Domain-0               0     0    0   -b-     193.1  any cpu

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>