Change the way that networks and PIFs relate to each other -- the PIF holds a

Change the way that networks and PIFs relate to each other -- the PIF holds a
reference to the network (and the host) not the other way around.  Persist only
the necessary details for the network (not the set of PIFs and VIFs).

Fix the plumbing of the network class inside XendAPI, and implement a few more
functions.

Signed-off-by: Ewan Mellor <ewan@xensource.com>