Libvirt: Adding video device to instances

Adding a video device driver to an instance, when VNC or SPICE are enabled.
Selecting the qxl driver when SPICE is enabled, otherwise use the Libvirt default, cirrus.

Implements blueprint libvirt-video-driver-selection
Change-Id: Icc3c538f2b81cc242baf87dc3538c27daf96ec38
Merge "Libvirt: Adding video device to instances"