lxc-ps: fix the display problem with arg --lxc

When we use arg --lxc to show processes in all containers, no
process displays, so fix it.

(Changelog: Serge: in-line fix of s/;;/;/ at line 69)

Signed-off-by: Weng Meiling <wengmeiling.weng@huawei.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>