vhost: add guest offload setting

vhost: add guest offload setting

Add guest offload setting in vhost lib.

Virtio 1.0 spec (5.1.6.4 Processing of Incoming Packets) says:

    1. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
       VIRTIO_NET_HDR_F_NEEDS_CSUM bit in flags can be set: if so,
       the packet checksum at offset csum_offset from csum_start
       and any preceding checksums have been validated. The checksum
       on the packet is incomplete and csum_start and csum_offset
       indicate how to calculate it (see Packet Transmission point 1).

    2. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were
       negotiated, then gso_type MAY be something other than
       VIRTIO_NET_HDR_GSO_NONE, and gso_size field indicates the
       desired MSS (see Packet Transmission point 2).

In order to support these features, the following changes are added,

1. Extend 'VHOST_SUPPORTED_FEATURES' macro to add the offload features negotiation.

2. Enqueue these offloads: convert some fields in mbuf to the fields in virtio_net_hdr.

There are more explanations for the implementation.

For VM2VM case, there is no need to do checksum, for we think the
  data should be reliable enough, and setting VIRTIO_NET_HDR_F_NEEDS_CSUM
  at RX side will let the TCP layer to bypass the checksum validation,
  so that the RX side could receive the packet in the end.

In terms of us-vhost, at vhost RX side, the offload information is
  inherited from mbuf, which is in turn inherited from TX side. If we
  can still get those info at RX side, it means the packet is from
  another VM at same host. So, it's safe to set the
  VIRTIO_NET_HDR_F_NEEDS_CSUM, to skip checksum validation.

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