Fix change of behaviour on execute introduced in #363
Fix change of behaviour on execute introduced in #363

Essentially, the behavior changed for when a caller to the
container's execute method wished to handle the packets received back on
the websocket.  This change fixes the reversion.

Signed-off-by: Alex Kavanagh <1997f654bcb4dfd26e564880ef69f22a14862003@canonical.com>

lxc/pylxd,lxc/pylxd