postcopy: Add notifier chain

postcopy: Add notifier chain

Add a notifier chain for postcopy with a 'reason' flag
and an opportunity for a notifier member to return an error.

Call it when enabling postcopy.

This will initially used to enable devices to declare they're unable
to postcopy and later to notify of devices of stages within postcopy.

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>