block: Use bdrv_has_zero_init_truncate()

vhdx and parallels call bdrv_has_zero_init() when they do not really
care about an image's post-create state but only about what happens when
you grow an image.  That is a bit ugly, and also overly safe when
growing preallocated images without preallocating the new areas.

Let them use bdrv_has_zero_init_truncate() instead.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190724171239.8764-6-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
[mreitz: Added commit message]
Signed-off-by: Max Reitz <mreitz@redhat.com>