[libvirt] --disable-nls is ignored in master

It seems some of the recent changes for 'po' broke the --disable-nls configure option. I expect no locales will be installed with --disable-nls, but they are installed anyway. This leads to the well known "error: Installed (but unpackaged) file(s) found:" failure. I tested libvirt-20180420T081114.3c66d5108. Olaf