bdrv_has_zero_init() only has meaning for newly created images or image
areas.  If qemu-img convert did not create the image itself, it cannot
rely on bdrv_has_zero_init()'s result to carry any meaning.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190724171239.8764-2-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>