i386: Add ordering field to CPUClass

Instead of using kvm_enabled to order the "-cpu help" list, use a
new "ordering" field for that.

Message-Id: <20170119210449.11991-3-ehabkost@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>