CA-268279: Fix self-conflicting network purposes
CA-268279: Fix self-conflicting network purposes

If any network had any purpose, this was being treated as a
conflict with the addition of any purpose to any network.

Signed-off-by: Thomas Sanders <c547603d2dd860735c4f0c043b1348f5c48c02f8@citrix.com>

srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api