qmp: add command "x-oob-test"

qmp: add command "x-oob-test"

This command is only used to test OOB functionality.  It should not be
used for any other purposes.

Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Peter Xu <[email protected]>
Message-Id: <20180309090006[email protected]>
Reviewed-by: Eric Blake <[email protected]>
[eblake: grammar tweak]
Signed-off-by: Eric Blake <[email protected]>