virtio: use zeroed memory for simple Tx header

virtio: use zeroed memory for simple Tx header

For simple TX the virtio-net header must be zeroed, but it was using memory
that had been initialized with indirect descriptor tables. This resulted in
"unsupported gso type" errors from librte_vhost.

We can use the same memory for every descriptor to save cachelines in the
vswitch.

Fixes: 6dc5de3a ("virtio: use indirect ring elements")

Signed-off-by: Rich Lane <rich.lane@bigswitch.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Jianfeng Tan <jianfeng.tan@intel.com>