net: remove implicit peer from offload API

The virtio_net offload APIs are used on the NIC's peer (i.e. the tap
device).  The API was defined to implicitly use nc->peer, saving the
caller the trouble.

This wasn't ideal because:
1. There are callers who have the peer but not the NIC.  Currently they
   are forced to bypass the API and access peer->info->... directly.
2. The rest of the net.h API uses nc, not nc->peer, so it is
   inconsistent.

This patch pushes nc->peer back up to callers.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
