docs: Introduce $(devhelphtml_generated)

docs: Introduce $(devhelphtml_generated)

Our XSLT magic generates one Devhelp-compatible HTML file
per documentation module, but so far we have only shipped
and installed documentation for virterror.

Now that we have $(modules), however, we can generate the
list of files the same way we do for regular documentation
and make sure we always ship and install everything.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>