e1000: NetClientInfo.receive_iov implemented

This patch implements the NetClientInfo.receive_iov method for the
e1000 device emulation. In this way a network backend that uses
qemu_sendv_packet() can deliver the fragmented packet without
requiring an additional copy in the frontend/backend network code
(nc_sendv_compat() function).

The existing method NetClientInfo.receive has been reimplemented
using the new method.

Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>