Specifying ':' to suppress the error messages printed by getopt().
Then, distinguish the two types of errors.

Before:
 # virsh -c
   virsh: option requires an argument -- 'c'
   error: unsupported option '-?'. See --help.

After:
 # virsh -c
   error: option '-c' requires an argument

 # virsh -x
   error: unsupported option '-x'. See --help.