examples/vhost: add virtio offload

examples/vhost: add virtio offload

Change the codes in vhost sample to test virtio offload feature.

These changes include,

1. add two test options: tx-csum and tso.

2. add virtio_tx_offload() function to test vhost TX offload feature
   for VM to NIC case;

however, for VM to VM case, it doesn't need to call this function,
  the reason is explained in patch 2.

Signed-off-by: Jijiang Liu <jijiang.liu@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>