net/virtio: add Tx preparation

net/virtio: add Tx preparation

Virtio requires pseudo-header checksum in TCP/UDP checksum to do
offload, but it was lost when Tx prepare is introduced. Also
rte_validate_tx_offload() should be used to validate Tx offloads.

Also it is incorrect to do virtio_tso_fix_cksum() after prepend
to mbuf without taking prepended size into account, since layer 2/3/4
lengths provide incorrect offsets after prepend.

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>