tests: qmp-test: add oob test

tests: qmp-test: add oob test

Test the new OOB capability.  Here we used the new "x-oob-test" command.
First, we send a lock=true and oob=false command to hang the main
thread.  Then send another lock=false and oob=true command (which will
be run inside parser this time) to free that hanged command.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-24-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweaks]
Signed-off-by: Eric Blake <eblake@redhat.com>