lxc_cgroup_path_get currently cache the cgroup mount point plus the
container name at the same time, making every call of the function
returning the same value.
It mean that actually every call to lxc_cgroup_get with a different
container name will in fact use the same container name as used for
the primary call.

I join a patch to fix that, still doing some caching, but only caching
the cgroup moint point this time. This patch actually work for me, as
I am using the liblxc for retrieving statistics about all running
containers, using lxc_cgroup_get to retrieve every interesting values.
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>