LXC from native: map vlan network type

The problem with VLAN is that the user still has to manually create the
vlan interface on the host. Then the generated configuration will use
it as a nerwork hostdev device. So the generated configurations of the
following two fragments are equivalent.

lxc.network.type = phys
lxc.network.link = eth0.5

lxc.network.type = vlan
lxc.network.link = eth0
lxc.network.vlan.id = 5