$ ls /usr/share/doc/libvirt-1.2.21* /usr/share/doc/libvirt-1.2.21: html /usr/share/doc/libvirt-1.2.21-r1: README.gentoo.bz2 ebuild fixed for me by replacing in src_configure: --htmldir=/usr/share/doc/${PF}/html with: --with-html-subdir=${PF}/html --docdir does not appear to be used.
commit f32344a9dca857c80a9e1382e838c2ffc573074f Author: Matthias Maier <tamiko@gentoo.org> Date: Sat Dec 26 22:27:44 2015 +0100 app-emulation/libvirt: Install all docs under /usr/share/doc/${PF} (bug 569526) Gentoo-Bug: 569526 Package-Manager: portage-2.2.26 Signed-off-by: Matthias Maier <tamiko@gentoo.org>