ses: Move cts list from local session to generic session

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
ct: Move list-head from local container to container handle

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
pbunix: Don't create another proxy for ct with existing one

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
test: Add test for handles opening

And fix two BUGs() found :

* free old proxies on unix session close
* don't search for existing proxies with malloc-ed variable

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>