cpu: x86: Add support for adding features to existing CPU models

cpu: x86: Add support for adding features to existing CPU models

This is not a good idea in general, but we can (and have to) do it in
specific cases when a feature has always been part of a CPU model in
hypervisor's definition, but we ignored it and did not include the
feature in our definition.

Blindly adding the features to the CPU map and not adding them to
existing CPU models breaks migration between old and new libvirt in both
directions. New libvirt would complain the features got unexpectedly
enabled (as they were not mentioned in the incoming domain XML) even
though they were also enabled on the source and the old libvirt just
didn't know about them. On the other hand, old libvirt would refuse to
accept incoming migration of a domain started by new libvirt because the
domain XML would contain CPU features unknown to the old libvirt.

This is exactly what happened when several vmx-* features were added a
few releases back. Migration between libvirt releases before and after
the addition is now broken.

This patch adds support for added these features to existing CPU models
by marking them with added='yes'. The features will not be considered
part of the CPU model and will be described explicitly via additional
<feature/> elements, but the compatibility check will not complain if
they are enabled by the hypervisor even though they were not explicitly
mentioned in the CPU definition and incoming migration from old libvirt
will succeed.

To fix outgoing migration to old libvirt, we also need to drop all those
features from domain XML unless they were explicitly requested by the
user. This will be handled by a later patch.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>