i386: Make unversioned CPU models be aliases

This will make unversioned CPU models behavior depend on the
machine type:

* "pc-*-4.0" and older will not report them as aliases.
  This is done to keep compatibility with older QEMU versions
  after management software starts translating aliases.

* "pc-*-4.1" will translate unversioned CPU models to -v1.
  This is done to keep compatibility with existing management
  software, that still relies on CPU model runnability promises.

* "none" will translate unversioned CPU models to their latest
  version.  This is planned become the default in future machine
  types (probably in pc-*-4.3).

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190628002844.24894-8-ehabkost@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>