supermin: Prevent multilib corruption (RHBZ#558593).

On some combination of installing, upgrading and removing the base
libguestfs package on x86_64, multilib can corrupt libguestfs by
leaving a copy of /usr/bin/libguestfs-supermin-helper around which
references the wrong architecture (usually, contains links to the
i386-based appliance, when the x86_64 appliance should be constructed).

This commit changes libguestfs-supermin-helper so that the script is
the same on all architectures.  Instead, the library passes the
differences to the script (eg. $host_cpu).  Because the i386 and
x86_64 libraries should be at different locations (/usr/lib vs
/usr/lib64) this should prevent multilib from screwing things up.

Related links:
https://bugzilla.redhat.com/show_bug.cgi?id=558593
http://rwmj.wordpress.com/2009/11/16/please-someone-shoot-multilib/#content
https://bugzilla.redhat.com/show_bug.cgi?id=235752