i386: Create "max" CPU model

Rename the existing "host" CPU model to "max, and set it to
kvm_enabled=false. The new "max" CPU model will be able to enable
all features supported by TCG out of the box, because its logic
is based on x86_cpu_get_supported_feature_word(), which already
works with TCG.

A new KVM-specific "host" class was added, that simply inherits
everything from "max" except the 'ordering' and 'description'
fields.

Message-Id: <20170222183919.11928-2-ehabkost@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>