d/libvirt-doc.docs: install new doc elements

I was comparing the versioned content that make install will put into
the debian/tmp build directory and what we have installed. We already
have a slight superset that I don't want to loose, but OTOH we also
have soem files missing that got added later.

Update debian/libvirt-doc.docs to include those extra files that we
miss.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>