blockdev: Mark {insert, remove}-medium experimental

blockdev: Mark {insert, remove}-medium experimental

While in the long term we want throttling to be its own block filter
BDS, in the short term we want it to be part of the BB instead of a BDS;
even in the long term we may want legacy throttling to be automatically
tied to the BB.

blockdev-insert-medium and blockdev-remove-medium do not retain
throttling information in the BB (deliberately so). Therefore, using
them means tying this information to a BDS, which would break the model
described above. (The same applies to other flags such as
detect_zeroes.) We probably want to move this information to the BB or
its own filter BDS before blockdev-{insert,remove}-medium can be
considered completely stable.

Therefore, mark these functions experimental for the time being.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1449847385-13986-2-git-send-email-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
[PMM: fixed format nit (underlining) in qmp-commands.hx]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>