NSX: allow net-migration only in combined mode

NSX: allow net-migration only in combined mode

To allow migration of networks from the agent based model
to the agentless one, both modes need to be available; prior
to this change the extension was available in 'agentless' mode
too; this may lead to leftovers during the migration step.
This patch makes the extension available in 'combined' mode
only, which was the intended behavior.

Change-Id: I491ee3eaf25fc30c0ab4092ee198802a6b06d91d
Closes-bug: #1313985
(cherry picked from commit e737076c374d0743927782b6318132ecf226f05a)