target-i386: Disable cache info passthrough by default

The host cache information may not make sense for the guest if the VM
CPU topology doesn't match the host CPU topology. To make sure we won't
expose broken cache information to the guest, disable cache info
passthrough by default, and add a new "host-cache-info" property that
can be used to enable the old behavior for users that really need it.

Cc: Benoît Canet <benoit@irqsave.net>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
