Libvirt VIF Driver Enhacement

The Libvirt VIF driver implementations as present in Folsom suffer from a number of problems

The Nova admin has to know what Quantum driver is used and configure the vif_driver to match
Nova can only support a single VIF type for Quantum, which prevents concurrent use of different Quantum drivers with Nova
The Nova admin is at risk of making suboptimal config choices. For example, choosing LibvirtOpenVswitchDriver instead of LibvirtOpenVswitchVirtualPortDriver for new libvirt
The Nova admin is at risk of making nonsensical config choices. For example, choosing LibvirtOpenVswitchDriver at the same time as the Libvirt NWFilter firewall will result in non-operational firewall - they should use LibvirtHybridOVSBridgeDriver if they need firewalling to work

Current VIF Driver Impls in Folsom

As of Folsom the following VIF drivers exist

[[LibvirtBridgeDriver]](LibvirtBaseVIFDriver). Connects VIFs to a plain Linux software bridge, whose name is provided by the network['bridge'] parameter. Configures filter parameters for nwfilter firewalling
[[LibvirtOpenVswitchDriver]](LibvirtBaseVIFDriver)
LibvirtHybridOVSBridgeDriver([[LibvirtBridgeDriver]], [[LibvirtOpenVswitchDriver]])
[[LibvirtOpenVswitchVirtualPortDriver]]([[LibvirtOpenVswitchDriver]])
QuantumLinuxBridgeVIFDriver(LibvirtBaseVIFDriver)