allow lxc.network.pair to specify host-side name for veth interface

allow lxc.network.pair to specify host-side name for veth interface

Currently we allocate veth device with random name on host side,
so that things like firewall rules or accounting does not work
at all.  Fix this by recognizing yet anothe keyword to specify
the host-side device name: lxc.network.pair, and use it instead
of random name if specified.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>