i386: Disable TOPOEXT by default on "-cpu host"

i386: Disable TOPOEXT by default on "-cpu host"

Enabling TOPOEXT is always allowed, but it can't be enabled
blindly by "-cpu host" because it may make guests crash if the
rest of the cache topology information isn't provided or isn't
consistent.

This addresses the bug reported at:
https://bugzilla.redhat.com/show_bug.cgi?id=1613277

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20180809221852.15285-1-ehabkost@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
(cherry picked from commit 7210a02c58572b2686a3a8d610c6628f87864aed)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>