virtio::net::txq: Use proper size of virtio_net_hdr

According to virtio specification virtio_net_hdr should include
num_buffers field only if VIRTIO_NET_F_MRG_RXBUF feature is negotiated.
Although this field is not used in the Tx flow its presence impacts
the size of the whole virtio_net_hdr struct. Therefore it should be taken
into an account when posting a virtio_net_hdr in a Tx flow.

The above has been broken by commit f99c90b9 and this patch fixes it back.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Reported-by:   Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>