XenAPI: Fix neutron installation

Several changes to support neutron setup with XenServer:
 - Always create an integration network. Before this change, the network
   was only created if neutron enabled. As the `is_service_enabled`
   function is not available, the condition always evaluated to false
   leaving the user without the required network.
 - Move openvswitch agent's xapi plugin installation to domU. Use the
   same mechanism as used for installing the nova plugins.
 - Remove non-used functions, non-used dummy values.

Change-Id: I8a30856374de74b06f155dbcfbd48b9d8dc12c64