migration: Deprecate redundant query-migrate result @blocked

migration: Deprecate redundant query-migrate result @blocked

Result @blocked is true when and only when result @blocked-reasons is
present.  It's always non-empty when present.  @blocked is redundant.
It was introduced in commit 3af8554bd0 "migration: Add blocker
information", and has not been released.  This gives us a chance to
fix the interface with minimal fuss.

Unfortunately, we're already too close to the release to risk dropping
it.  Deprecate it instead.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Message-id: 20210420051907[email protected]
Signed-off-by: Peter Maydell <[email protected]>