interface-reconfigure: callout to datapath backend class method on rewrite

interface-reconfigure: callout to datapath backend class method on rewrite

Use this mechanism to allow the vswitch backend to update the vswitch
configuration's mapping from datapath to XenAPI datamodel Network
UUIDs. The vswitch needs a mechanism to update these when they change
(i.e. on pool join and eject).

Refactor the DatapathFactory method to return the class which the
caller can instantiate or not as the require.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>