lxc-user-nic: keep lines from other {users,links}

lxc-user-nic: keep lines from other {users,links}

Assume the db contained the following entries:

    chb veth lxcbr0 veth1
    chb veth lxcbr0 veth2
    chb veth lxdbr0 veth3
    chb veth lxdbr0 veth2
    didi veth lxcbr0 veth4

And you request

    cull_entries("chb", "veth", "lxdbr0", "veth3");

lxc-user-nic would wipe any entries that did not match irrespective of whether
they existed or not. Let's fix that.

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