net: Treat venet device as a separate complete device

When restoring venet device we need to restore its
index as well, which actually possible with new iproute2
package but the problem is that the index itself lays
inside image file. We could use crit tool to extract
it but this would slowdon procedure signifantly (need
to run python which would parse the image, or need
to pass the index into environmnet from inside of
the CRIU itself).

So lets do a trick and simply created venet device
inside container by criu itself (thanks we support
creating venet via netlink interface now).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>