[libvirt] avoid "make rpm" failure in doc/

[Jim Meyering]

Jim Meyering wrote:

> "make rpm" fails like this:
>
>     Rebuilding the HTML pages from the XML API
>     make[3]: *** No rule to make target `html/libvirt-libvirt.html', needed by `distdir'.  Stop.
>     make[3]: *** Waiting for unfinished jobs....
>     Validating the resulting XHTML pages
>
> These two patches fix that:
> (though "make rpm" still fails for me:
>
>   extracting debug info from /j/rpmbuild/BUILDROOT/libvirt-0.7.6-1.fc12.x86_6h..
>   *** ERROR: No build ID note found in /j/rpmbuild/BUILDROOT/libvirt-0.7.6-1.h..
>
> The first (1/2) change is not officially required to solve this problem,
> but it overlapped, and does fix a non-distdir build failure.

FYI, I've just pushed these and the unused-var fix.
Of course, if anything comes out in review, I'll deal with it.