i386: Create "max" CPU model

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[email protected]>
Tested-by: Richard W.M. Jones <[email protected]>
Tested-by: Jiri Denemark <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>