qemu-img: conditionally zero out target on convert
If the target has_zero_init = 0, but supports efficiently writing zeroes by unmapping we call bdrv_make_zero to avoid fully allocating the target. This currently works only for iscsi. It can be extended to raw with BLKDISCARDZEROES for example. Reviewed-by: NEric Blake <eblake@redhat.com> Signed-off-by: NPeter Lieven <pl@kamp.de> Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
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