allow lxcapi_get_cgroup_item() on lxc-execute containers

Containers started with lxc-execute may not have a conf, but
nothing in the implementation of lxcapi_get_cgroup_item()
actually needs/uses it, and it can be useful to get items out
of the containers' cgroup items.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
fix leak when a veth.pair name is specified

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>