Neutron with XenServer properly doesn't close XenAPI sessions.
If it creates these sessions so rapidly, the XenServer host eventually
exceeds its maximum allowed number of connections.
This patch adds a close process for session.
Closes-Bug: 1558721
Change-Id: Ida90a970c649745c492c28c41c4a151e4d940aa6
(cherry picked from commit 9d21b5ad7e)