Add support to change inteface model from virtio

Add e1000, pcnet, rtl8139, and ne2k_pci to choices for model and test to
 ensure it updates.

This is needed to help with testing alternate drivers that sometimes need to
 be tested for errors.

Closes-bug #1257513
Merge pull request #25 from xarses/add_inteface_models

Add support to change inteface model from virtio
change video driver
Update libvirt_xml_builder.py
Merge pull request #28 from monroid/video_vga

change video driver