Use container specific domain socket name

Otherwise trying to start N containers in parallel gives "lxc_container: bind : Address already in use" error.

Found while using Go bindings to create/start/stop large number of containers in parallel so I reproduced the same using Python API to rule out possible Go related problems.

----
import lxc

from queue import Empty
from multiprocessing import Process, Queue

def create(q):
    while True:
        try:
            x = q.get(block=False)
            c = lxc.Container(str(x))
            print("Starting", x)
            c.start()
        except Empty:
            break

if __name__=="__main__":
    work_queue = Queue()
    for i in range(11):
        work_queue.put(i)

    process = [Process(target=create, args=(work_queue,)) for i in range(4)]
    for p in process:
        p.start()

    for p in process:
        p.join()
----

caglar@qgq:~/Project$ sudo python3 cstart.py
Starting 0
Starting 1
lxc_container: bind : Address already in use
Starting 3
Starting 4
Starting 2
lxc_container: lxc_container: bind : Address already in use
bind : Address already in uselxc_container: bind : Address already in use

Starting 5
Starting 6
lxc_container: bind : Address already in use
lxc_container: bind : Address already in use
Starting 7
Starting 8
Starting 9
lxc_container: bind : Address already in use
lxc_container: bind : Address already in use
lxc_container: bind : Address already in use
Starting 10
lxc_container: bind : Address already in use

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>