Apparently, files in /usr/lib/sysctl.d are usually prefixed with numbers
for easier ordering. Let's be consistent with this. I chose 60 for
libvirtd so that it goes after 50-default.conf.
https://bugzilla.redhat.com/show_bug.cgi?id=1084876
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>