When not specifying a cluster size on the command line, qemu-img printed
a cluster size of 0:

    Formatting '/tmp/test.qcow2', fmt=qcow2 size=67108864
    encryption=off cluster_size=0

This patch adds the default cluster size to the QEMUOptionParameter list, so
that it displays the default value that is used.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>