net/virtio: move VLAN tag insertion to Tx prepare

net/virtio: move VLAN tag insertion to Tx prepare

VLAN tag insertion should be in Tx prepare, not in Tx burst functions.
One of Tx prepare goals is to be able to do preparations in advance
(possibly on different CPU core) and then transmit it fast.
Also Tx prepare can report that a packet does not pass Tx offloads
check. E.g. has no enough headroom to insert VLAN header.

Fixes: 4fb7e803eb1a ("ethdev: add Tx preparation")
Cc: stable@dpdk.org
Signed-off-by: Dilshod Urazov <dilshod.urazov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>