virtio-net: correctly report maximum tx_queue_size value

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.

The value is returned by virtio_net_max_tx_queue_size() to set the
parameter:

    n->net_conf.tx_queue_size = MIN(virtio_net_max_tx_queue_size(n),
                                    n->net_conf.tx_queue_size);

But the parameter checking uses VIRTQUEUE_MAX_SIZE (1024).

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

   ... -netdev tap,... -device virtio-net,tx_queue_size=1024

    # ethtool -g enp0s2
    Ring parameters for enp0s2:
    Pre-set maximums:
    RX:		256
    RX Mini:	n/a
    RX Jumbo:	n/a
    TX:		256
    Current hardware settings:
    RX:		256
    RX Mini:	n/a
    RX Jumbo:	n/a
    TX:		256

   ... -netdev vhost-user,... -device virtio-net,tx_queue_size=2048

    Invalid tx_queue_size (= 2048), must be a power of 2 between 256 and 1024

With this patch the correct maximum value is checked and displayed.

For vDPA/vhost-user:

    Invalid tx_queue_size (= 2048), must be a power of 2 between 256 and 1024

For all the others:

    Invalid tx_queue_size (= 512), must be a power of 2 between 256 and 256

Fixes: 2eef278b9e63 ("virtio-net: fix tx queue size for !vhost-user")
Cc: mst@redhat.com
Cc: qemu-stable@nongnu.org
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>