qapi: Mark unstable QMP parts with feature 'unstable'

Add special feature 'unstable' everywhere the name starts with 'x-',
except for InputBarrierProperties member x-origin and
MemoryBackendProperties member x-use-canonical-path-for-ramblock-id,
because these two are actually stable.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Acked-by: John Snow <jsnow@redhat.com>
Message-Id: <20211028102520.747396-3-armbru@redhat.com>

qapi/misc.json

@@ -185,6 +185,9 @@
 # available during the preconfig state (i.e. when the --preconfig command
 # line option was in use).
 #
+# Features:
+# @unstable: This command is experimental.
+#
 # Since 3.0
 #
 # Returns: nothing
@@ -195,7 +198,8 @@
 # <- { "return": {} }
 #
 ##
-{ 'command': 'x-exit-preconfig', 'allow-preconfig': true }
+{ 'command': 'x-exit-preconfig', 'allow-preconfig': true,
+  'features': [ 'unstable' ] }
 
 ##
 # @human-monitor-command: