Make lxc-user-nic use mkifname

NetworkManager at least expects all veth devices to be called veth*
otherwise it'll consider them as physical interface and try to do DHCP
on them.

This change makes lxc-user-nic use the same function that we use for LXC
itself which will give us standard vethXXXXX kind of interfaces.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>