virt-ssh-helper: Don't use optind

virt-ssh-helper: Don't use optind

It's a getopt interface and we're not using getopt, at least
directly, so even though it works relying on it feels wrong.

GOption takes care of removing any trace of the arguments it
consumes from argc and argv, leaving behind only non-option
arguments, so we can just use those standard variables.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>