[RFC PATCH v5 ] virtio-spec: Introduce new Receive-Segment-Coalescing feature for guest

Hi There,
This is the updated specification for Receive Segment Coalescing feature.

Changelog:
v4->v5
-- Redefine support of the feature bit from
   by 'VIRTIO_NET_F_CTRL_GUEST_OFFLOADS' feature bit to
   VIRTIO_NET_GUEST_RSC4/6.
-- Add varying of the header fields description in case of
   if or not the feature is negotiated.
-- Add supporting of dynamically control of the feature by
   'VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET' control command.


v3->v4
-- Remove maximum packets size section.