qemu: domain: Drop added features from migratable CPU

qemu: domain: Drop added features from migratable CPU

Features marked with added='yes' in CPU model definitions have to be
removed before migration, otherwise older libvirt would complain about
unknown CPU features. We only do this for features that were enabled for
a given CPU model even with older libvirt, which just ignored the
features. And only for features we added ourselves when updating CPU
definition during domain startup, that is we do not remove features
which were explicitly mentioned by a user.

That said, this is not the safest thing we could do, but it's
effectively the same thing we did before the affected features were
added: we ignored them completely on both sides of migration.

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