net/virtio-user: enable offloading

net/virtio-user: enable offloading

When used with vhost kernel backend, we can offload at both directions.
  - From vhost kernel to virtio_user, the offload is enabled so that
    DPDK app can trust the flow is checksum-correct; and if DPDK app
    sends it through another port, the checksum needs to be
    recalculated or offloaded. It also applies to TSO.
  - From virtio_user to vhost_kernel, the offload is enabled so that
    kernel can trust the flow is L4-checksum-correct, no need to verify
    it; if kernel will consume it, DPDK app should make sure the
    l3-checksum is correctly set.

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