webadmin: Override getEntity() in HostSetupNetworksModel

It always returns VDS, so no reason to cast every time it's called.

Change-Id: I9e879938d5d3b80d680aa759cc172f957ba77571
Bug-Url: https://bugzilla.redhat.com/1008999
Signed-off-by: Lior Vernia <lvernia@redhat.com>