spec: fix logic bug in deciding to turn on cgconfig

spec: fix logic bug in deciding to turn on cgconfig

https://bugzilla.redhat.com/show_bug.cgi?id=738725

Commit ecd8725 tried to silence a spurious warning on the initial
libvirt install, and commit ba6cbb1 tried to fix up the logic to the
correct Fedora version, but the warning was still present due to a
logic bug: since %{fedora} and %{rhel} are never simulatanously
set, then 0%{rhel} <= 6 made the %if always true.  Checking for
minimum versions (via >=) is okay, but checking for maximum versions
(via <=) requires a prerequisite test that the platform being tested
is non-zero.

Also fix a bogus setting of with_libxl (although we previously
hard-code with_libxl to 0 for rhel earlier in the file, so this
was not as severe a bug).

* libvirt.spec.in (with_cgconfig): Don't enable cgconfig on F16.