Make image props filter handle old vm_modes

The ImagePropertyFilter would fail to build an instance if the image
used an older style vm_mode (e.g. 'pv') as described in
nova.compute.vm_mode.  This change will convert the vm mode string to
the current form.

Change-Id: Ibde37e28b02d9e866b286ea14c47a6a8411d94b4
Closes-Bug: #1276773
Merge "Make image props filter handle old vm_modes"