The network driver is not doing correct checking for
duplicate UUID/name values. This introduces a new method
virNetworkObjIsDuplicate, based on the previously
written virDomainObjIsDuplicate.

* src/conf/network_conf.c, src/conf/network_conf.c,
  src/libvirt_private.syms: Add virNetworkObjIsDuplicate,
* src/network/bridge_driver.c: Call virNetworkObjIsDuplicate
  for checking uniqueness of uuid/names