qapi: Mark unstable QMP parts with feature 'unstable'

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 <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Acked-by: John Snow <[email protected]>
Message-Id: <20211028102520[email protected]>