qapi/qom: Document feature unstable of @x-vfio-user-server

qapi/qom: Document feature unstable of @x-vfio-user-server

Commit 8f9a9259d32c added ObjectType member @x-vfio-user-server with
feature unstable, but neglected to explain why it is unstable.  Do
that now.

Fixes: 8f9a9259d32c (vfio-user: define vfio-user-server object)
Cc: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Cc: John G Johnson <john.g.johnson@oracle.com>
Cc: Jagannathan Raman <jag.raman@oracle.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240703095310.1242102-1-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
[Indentation fixed]
(cherry picked from commit 3becc939081097ccfed6968771f33d65ce8551eb)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>