qapi-schema: Comment about full expansion of non-migration-safe models

Add a note warning that static expansion may not be 100% accurate
when the CPU model is not migration-safe. This will be the case
on x86 when expansing the "host" CPU model, because there are
"host" features that can't have a migration-safe representation
(e.g. "host-cache-info").

Message-Id: <20170116211124.29245-3-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>