libvirt: Enable custom video RAM setting

hw_video_ram can be provided via the image properties in Glance.
The value should be provided in MB.
hw_video_ram will be honoured, during an instance creation,
only if a coresponding hw_video:ram_max_mb value has been set in the flavor's
extra_specs and it's higher than the requested amount in hw_video_ram.

    glance image-update \
       --property hw_video_model=qxl \
       --property hw_video_ram=64 \
       [image]

DocImpact
Partially implements blueprint libvirt-video-driver-selection

Change-Id: I125cdc2afa27986ead9a81d925125a6a3ecce145
Merge "libvirt: Enable custom video RAM setting"