qemu: wire BDRV_REQ_NO_FALLBACK
to NBD_CMD_FLAG_FAST_ZERO in both server and client
libnbd: add ENOTSUP errno,
add LIBNBD_CMD_FLAG_FAST_ZERO to nbd_zero()
and nbd_aio_zero()
nbdkit: add support in the following filters and plugins: always fails always fast conditional