cfg2cmd: netdev: fix value for tx_queue_size

cfg2cmd: netdev: fix value for tx_queue_size

Quoting from QEMU commit 4271f40383 ("virtio-net: correctly report
maximum tx_queue_size value"):

> Maximum value for tx_queue_size depends on the backend type.
> 1024 for vDPA/vhost-user, 256 for all the others.

> So the parameter is silently ignored and ethtool reports a different
> value than the one provided by the user.

Indeed, for a non-vDPA/vhost-user netdev, the guest will see TX: 256
instead of the specified 1024 here. With the mentioned QEMU commit (in
master and will be part of 8.1), using 1024 will be a hard error:

> Invalid tx_queue_size (= 1024), must be a power of 2 between 256 and 256

Since neither vhost-user, nor vhost-vdpa netdev types are exposed by
Proxmox VE, just changing the limit to the correct 256 should be fine.
No obvious issue during live-migration found.

Fixes: 620d6b32 ("virtio-net: increase defaults rx|tx-queue-size to 1024")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>