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>