qemu_storage: rename image_cluster_size to cluster_size in QemuImg.create()

Sync internal code to upstream.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu_storage: Add QemuImg.info() function

We need this function in some test case.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu_storage: Make 'cluster_size= ' could disable this parameter.

Change 'if cluster_size is not None' to 'if cluster_size'.
Then we can use 'cluster_size= ' to disable pre set cluster_size
parameter.
This feature is useful when we want to disable pre-set cluster_size
parameter.

Signed-off-by: Feng Yang <fyang@redhat.com>