Refactor network_utils to new call_xenapi pattern

Move network_utils to the new call_xenapi style, and add some unit
tests, and to make the unit reasonable, start raising NovaException.

Change-Id: I991965ffd57bf4b309cac8c6e9d4ef94f8d430f2
Merge "Refactor network_utils to new call_xenapi pattern"
Merge commit 'refs/changes/38/84138/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch