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 <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 20210420051907.891470-1-armbru@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

qapi/migration.json

@@ -224,9 +224,14 @@
 #        only returned if VFIO device is present, migration is supported by all
 #        VFIO devices and status is 'active' or 'completed' (since 5.2)
 #
+# @blocked-reasons: A list of reasons an outgoing migration is blocked.
+#                   Present and non-empty when migration is blocked.
+#                   (since 6.0)
+#
 # @blocked: True if outgoing migration is blocked (since 6.0)
 #
-# @blocked-reasons: A list of reasons an outgoing migration is blocked (since 6.0)
+# Features:
+# @deprecated: Member @blocked is deprecated.  Use @blocked-reasons instead.
 #
 # Since: 0.14
 ##
@@ -241,7 +246,7 @@
            '*setup-time': 'int',
            '*cpu-throttle-percentage': 'int',
            '*error-desc': 'str',
-           'blocked': 'bool',
+           'blocked': { 'type': 'bool', 'features': [ 'deprecated' ] },
            '*blocked-reasons': ['str'],
            '*postcopy-blocktime' : 'uint32',
            '*postcopy-vcpu-blocktime': ['uint32'],