Add hw_vif_model and hw_disk_bus to glance properties

Document hw_vif_model (for both libvirt and VMware) and hw_disk_bus
(libvirt) based on information in nova repository files
nova/virt/libvirt/vif.py and nova/virt/libvirt/vif.py.

Change-Id: Ib22c8192a60b35fb1bb8c755c4d9bff9c72456f7
Closes-Bug: #1314738
Merge "Add hw_vif_model and hw_disk_bus to glance properties"