Neutron LBaaS allows deleting a listener that still has default
pool. In that case, we should update the virtual server in the NSX
backend to remove the pool_id. Otherwise, the backend will throw
exception. This patch fixes that by updating the virtual server
before deleting it if listner has pool.
Change-Id: I6c7a252423b65d8f6a0c562b5508a1488c159dba
d3df8e4bd0