migration/block: use blk_pwrite_zeroes for each zero cluster

BLOCK_SIZE is (1 << 20), qcow2 cluster size is 65536 by default,
this may cause the qcow2 file size to be bigger after migration.
This patch checks each cluster, using blk_pwrite_zeroes for each
zero cluster.

[Initialize cluster_size to BLOCK_SIZE to prevent a gcc uninitialized
variable compiler warning.  In reality we always initialize cluster_size
in a conditional but gcc doesn't know that.
--Stefan]

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Lidong Chen <lidongchen@tencent.com>
Message-id: 1492050868-16200-1-git-send-email-lidongchen@tencent.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
