Provides two drivers for implementing security groups in xenapi:
1) domU driver that enforces security groups on the Openstack virtual appliance (use advised with FlatDHCP in HA mode)
2) dom0 driver that enforces security groups where VIFs are attached
Both drivers translate security groups into iptables rules.
Existing libvirt code has been refactored to reduce the amount of duplicated code to a minimum
Now Addressing reviewers's comments on style. Fixing issue spotted with snapshots
Change-Id: Ifa16a8f2508a709be03241bac0f942fe1a51d1e8