virtio/migration: Migrate virtio-net to VMState

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20170203160651.19917-5-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Merge fix against Halil's removal of the '_start' field in
    VMSTATE_VBUFFER_MULTIPLY