Libvirt: Making the video driver element configurable

hw_video_model can be provided via the image properties in Glance.
It may be set to one of the following values: "vga", "cirrus", "vmvga", "xen" or "qxl",
as these are currently being supported by libvirt.

    glance image-update \
       --property hw_video_model=qxl \
       [image]

DocImpact
Partially implements blueprint libvirt-video-driver-selection
Change-Id: Ie710eb9d8d094865d651373c6f9882f1ac12b49a
Merge "Libvirt: Making the video driver element configurable"