block: Declare blockdev-add and blockdev-del supported

It's been a long journey, but here we are.

The supported blockdev-add is not compatible to its experimental
predecessors; bump all Since: tags to 2.9.

x-blockdev-remove-medium, x-blockdev-insert-medium and
x-blockdev-change need a bit more work, so leave them alone for now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>