The 'block-stream' QMP command is documented in block-core.json but not
qmp-commands.hx.  Add a summary of the command to qmp-commands.hx
(similar to the documentation for 'block-commit').

Reported-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1429094622-26218-1-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>