net_pkt: Extend packet abstraction as required by e1000e functionality

This patch extends the TX/RX packet abstractions with features that will
be used by the e1000e device implementation.

Changes are:

  1. Support iovec lists for RX buffers
  2. Deeper RX packets parsing
  3. Loopback option for TX packets
  4. Extended VLAN headers handling
  5. RSS processing for RX packets

Signed-off-by: Dmitry Fleytman <dmitry.fleytman@ravellosystems.com>
Signed-off-by: Leonid Bloch <leonid.bloch@ravellosystems.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>