cgmanager: use absolute cgroup path to switch cgroups at attach

cgmanager: use absolute cgroup path to switch cgroups at attach

If an unprivileged user does 'lxc-start -n u1' in one
login session, followed by 'lxc-attach -n u1' in another
session, the attach will fail if the sessions are in different
cgroups.  The same is true of lxc-cgroup commands.

Address this by using the GetPidCgroupAbs and MovePidAbs
which work with the containers' cgroup path relative to
the cgproxy.

Since GetPidCgroupAbs is new to api version 3 in cgmanager,
use the old method if we are on an older cgmanager.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Tested-by: "S.Çağlar Onur" <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>