qemu_cpu.cfg: disable qemu-version-specific test cases by default

Instead of just defining rhel64 and qemu13 and enabling both by default,
define rhel64, qemu13, and unknown_qemu.

unknown_qemu will be enabled by default, so we don't test for things
that may fail on non-RHEL-6 or non-qemu-1.3 hosts. This should make the
tests work by default on most hosts, until we implement some kind of
host-based autoconfiguration system.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>