Replace virNetworkObjPtr with virNetworkDefPtr in network platform APIs
    
    The networkCheckRouteCollision, networkAddFirewallRules and
    networkRemoveFirewallRules APIs all take a virNetworkObjPtr
    instance, but only ever access the 'def' member. It thus
    simplifies testing if the APIs are changed to just take a
    virNetworkDefPtr instead
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>