genisoimage has been removed from centos-9, and xorrisofs creates a
symlink from /usr/bin/mkisofs to /usr/bin/xorrisofs. This means
bifrost's mkisofs fallback tasks will continue to work when xorrisofs
is installed.
Change-Id: I7ca056b102934f8906a43e0f9a202b6a5abc8c3b