network: allow lxc_network_move_by_index() rename netdev in moving.

network: allow lxc_network_move_by_index() rename netdev in moving.

In netlink, we can set the dest_name of netdev when move netdev
between namespaces in one netlink request. And moving a netdev of
a src_name to a netdev with a dest_name is a common usecase.

So this patch add a parametaer to lxc_network_move_by_index() to
indicate the dest_name for the movement. NULL means same with
the src_name.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>