block: use bdrv_has_zero_init to return BDRV_BLOCK_ZERO Alternatively, this could use a "discard zeroes data" flag returned by bdrv_get_info. Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>