network: implement lxc_get_netdev_by_idx()

network: implement lxc_get_netdev_by_idx()

lxc_get_netdev_by_idx() takes care of checking whether a given netdev struct
for a given index is already allocated. If so it returns a pointer to it to the
caller.
If it doesn't find it it will allocate a new netdev struct and insert it into
the network list at the right position. For example, let's say you have the
following networks defined in your config file:

lxc.network.0.type=veth
lxc.network.0.link=lxcbr0
lxc.network.0.name=eth0
lxc.network.0.flags=up

lxc.network.3.type=veth
lxc.network.3.link=lxcbr0
lxc.network.3.name=eth1
lxc.network.3.flags=up
lxc.network.3.hwaddr = 00:16:3e:5d:7d:65

lxc.network.1.type=veth
lxc.network.1.link=lxcbr0
lxc.network.1.name=eth0
lxc.network.1.flags=up

lxc_get_netdev_by_idx() will take care that the internal network list appears
in the order:

0
1
3

The ordering of the keys themselves can also be random, meaning you could do:

lxc.network.3.type=veth
lxc.network.0.link=lxcbr0
lxc.network.0.name=eth0
lxc.network.3.hwaddr = 00:16:3e:5d:7d:65
lxc.network.0.flags=up
lxc.network.3.flags=up
lxc.network.0.type=veth
lxc.network.3.link=lxcbr0
lxc.network.3.name=eth1

and lxc_get_netdev_by_idx() would still figure out the correct ordering since
it keeps track of the index.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>