Use libvirt filters for network security

Amend an earlier checkin.
Use net.get('shared') instead of net['shared'].
The later will raise KeyError if 'shared' is not found.

Closes-rally-bug: DE789 DE701 DE714 DE505
Implements: rally-task TA5323
Not-in-upstream: true
Merge pull request #32 from CiscoSystems/de711.2

Use libvirt filters for network security