migration: comment VMSTATE_UNUSED*() properly

migration: comment VMSTATE_UNUSED*() properly

It is error prone to use VMSTATE_UNUSED*() sometimes especially when
the size of the migration stream of the field is not the same as the
size of the structure (boolean is one example).  Comment it well so
people will be aware of this when people want to use it.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20190329095713.14177-1-peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>