fill missing netdev fields for unprivileged containers

lxc-user-nic now returns the names of the interfaces and
unpriv_assign_nic function parses that information to fill
missing netdev->veth_attr.pair and netdev->name.

With this patch get_running_config_item started to provide
correct information;

>>> import lxc; c = lxc.Container("rubik"); c.get_running_config_item("lxc.network.0.name"); c.get_running_config_item("lxc.network.0.veth.pair");
'eth0'
'veth9MT2L4'
>>>

and lxc-info started to show network stats;

lxc-info -n rubik
Name:           rubik
State:          RUNNING
PID:            23061
IP:             10.0.3.233
CPU use:        3.86 seconds
BlkIO use:      88.00 KiB
Memory use:     6.53 MiB
KMem use:       0 bytes
Link:           veth9MT2L4
 TX bytes:      3.45 KiB
 RX bytes:      8.83 KiB
 Total bytes:   12.29 KiB

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>