net: add ability to set names for outside links of veth devices

When restoring a container crtools create veth pair inside it and then
pushed one end to the namespaces crtools live in (outside). To facilitate
the subsequent management of the otter end of the veth pair this option
is added -- one can specifu a name by which the respective end would be
visible. E.g.: --veth-pair eth0=veth101.0

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>