make ctdladm(8) return 0 is everything was ok.

retval is used to test the return of XML_Parse function which is ok if 1 is
returned and retval it directly returned to the main function and used as an
exit value.

if all the parsing part is done reset retval to 0 so that the command return 0
if everything ok

Differential Revision: https://reviews.freebsd.org/D3102
Reviewed by: trasz
Sponsored by: gandi.net