[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The hvsupport.html.in file is constantly out of date, because when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