Fix instantiation of multiple vlan interfaces with same id

Fix instantiation of multiple vlan interfaces with same id

Container fail to start with configs (as shown below) where the same
vlan id is used for several type=vlan container interfaces.
Then, during the instantiation of the vlan interfaces, an error occurs
because the lxc code tries to assign the same temporary name to both
of them before it is bound into the container.

> lxc.network.type      = vlan
> lxc.network.flags     = up
> lxc.network.link      = eth1
> lxc.network.vlan.id   = 3842
> lxc.network.name      = iso0
>
> lxc.network.type      = vlan
> lxc.network.flags     = up
> lxc.network.link      = eth2
> lxc.network.vlan.id   = 3842
> lxc.network.name      = iso1

Signed-off-by: Axel Neumann <neumann@cgws.de>