block: Implement .bdrv_has_zero_init_truncate()

block: Implement .bdrv_has_zero_init_truncate()

We need to implement .bdrv_has_zero_init_truncate() for every block
driver that supports truncation and has a .bdrv_has_zero_init()
implementation.

Implement it the same way each driver implements .bdrv_has_zero_init().
This is at least not any more unsafe than what we had before.

Signed-off-by: Max Reitz <[email protected]>
Message-id: 20190724171239[email protected]
Reviewed-by: Maxim Levitsky <[email protected]>
Reviewed-by: Stefano Garzarella <[email protected]>
Signed-off-by: Max Reitz <[email protected]>