Drop libvirt hw device model code

The code here adds support for setting image properties to set the NIC
model or disk bus to e1000 or ide, respectively.  The code here likely
originated before upstream Nova had this support.  Nova added support
for this in the Grizzly release.  The image properties are hw_disk_bus
and hw_vif_model and support more options than the code that was here.

For more info on this support, you can find the blueprint here:

    https://blueprints.launchpad.net/nova/+spec/libvirt-custom-hardware