virsh: make -h always give help

virsh: make -h always give help

https://bugzilla.redhat.com/show_bug.cgi?id=817244 mentions that
unlike most other tools, where --help or --version prevent all
further parsing of all later options, virsh was strange in that
--version stopped parsing but --help tried to plow on to the end.
There was no rationale for this original implementation (since
2005!), so I think we can safely conform to common usage patterns.

* tools/virsh.c (main): Drop useless 'help' variable.