Libvirt: Always teardown lxc container on destroy

This fixes a bug where shutting down a libvirt-lxc based instance
would leak its underlying nbd device. This was happening because
_teardown_container would only get called if the domain was
present. After this fix, _teardown_container will always get called
on a destroy, which ensures the nbd device gets disconnected.

Change-Id: I8d0044fd038fef80568c6c86e34719ef679aa890
Closes-Bug: #1367363
Merge "Libvirt: Always teardown lxc container on destroy"