libvirt-l10n: New binary package

This new binary package contains just the localization for
libvirt and, while it gets installed by default due to libvirt0
recommending it, it can be removed if desired.

This brings the installed size of the libvirt0 package
from ~25 MiB to ~6 MiB and makes the
arch-dep-package-has-big-usr-share Lintian warning go away.