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 <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Message-id: 20130520081814[email protected]
Signed-off-by: Anthony Liguori <[email protected]>