virtio-net: dynamic network offloads configuration

virtio-net: dynamic network offloads configuration

Virtio-net driver currently negotiates network offloads
on startup via features mechanism and have no ability to
disable and re-enable offloads later.
This patch introduced a new control command that allows
to configure device network offloads state dynamically.
The patch also introduces a new feature flag
VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.

Signed-off-by: Dmitry Fleytman <dfleytma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 20130520081814.GA8162@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>