[libvirt] Allow hvsupport.html.in to be auto-generated

The hvsupport.html.in file is constantly out of date, because when updating the drivers to add new APIs, people often (always) forget to update the hvsupport.html.in file. To solve this we can instead store version number annotations in the drivers themselves, so it is not easily missed. Then the hvsupport.html.in file can be auto-generated