hvsupport: use a regex instead of XML::XPath

hvsupport: use a regex instead of XML::XPath

When generating the hvsupport.html.in file, we parse the -api.xml
files generated by apibuild.py to know in which HTML file the API
function is.

Doing an XPath query for every single 'function' element in the
file is inefficient.

Since the XML file is generated by another of our build scripts
(apibuild.py, using Python's standard 'output.write' XML library),
just find the function name->file mapping by a regex upfront.

Also add a note about this next to the line that generates it
in apibuild.py and do not check if XML::XPath is installed in
bootstrap since we no longer use it.