target-i386: turn off CPU.l3-cache only for 2.7 and older machine types

commit (14c985cf target-i386: present virtual L3 cache info for vcpus)
misplaced compat property putting it in new 2.8 machine type
which would effectively to disable feature until 2.9 is released.
Intent of commit probably should be to disable feature for 2.7
and older while allowing not yet released 2.8 to have feature
enabled by default.

Cc: qemu-stable@nongnu.org
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>