Also switch from cdn.debian.net to http.debian.net in xen-tools.conf
Do not run "passwd" via runCommand

runCommand captures STDOUT for logging which is not desired for
interactive commands.

Makes --passwd usable again.

Thanks to Christan Herzog for the bug report.
Loop around calling "passwd" to get a second chance to change the password

Needs Term::UI which is part of Debian's perl-modules package, hence
no new (build-) dependencies needed.