qemu: Add qemu-img cluster size check test

1) Create image without cluster_size option
2) Verify if the cluster_size is default value
3) Create image with cluster_size option
4) Verify if the cluster_size is the value set

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Acked-by: Qingtang Zhou <qzhou@redhat.com>