tests/qmp-test tests an out-of-band command overtaking a slow in-band
command.  To do that, it needs:

1. An in-band command that *reliably* takes long enough to be
   overtaken.

2. An out-of-band command to do the overtaking.

3. To avoid delays, a way to make the in-band command complete quickly
   after it was overtaken.

To satisfy these needs, commit 469638f9 provides the rather
peculiar oob-capable QMP command x-oob-test:

* With "lock": true, it waits for a global semaphore.

* With "lock": false, it signals the global semaphore.

To satisfy 1., the test runs x-oob-test in-band with "lock": true.
To satisfy 2. and 3., it runs x-oob-test out-of-band with "lock": false.

Note that waiting for a semaphore violates the rules for oob-capable
commands.  Running x-oob-test with "lock": true hangs the monitor
until you run x-oob-test with "lock": false on another monitor (which
you might not have set up).

Having an externally visible QMP command that may hang the monitor is
not nice.  Let's apply a little more ingenuity to the problem.  Idea:
have an existing command block on reading a FIFO special file, unblock
it by opening the FIFO for writing.

For 1., use

    {"execute": "blockdev-add",  "id": ID1,
     "arguments": {
        "driver": "blkdebug", "node-name": ID1, "config": FIFO,
        "image": { "driver": "null-co"}}}

where ID1 is an arbitrary string, and FIFO is the name of the FIFO.

For 2., use

    {"execute": "migrate-pause", "id": ID2, "control": {"run-oob": true}}

where ID2 is a different arbitrary string.  Since there's no migration
to pause, the command will fail, but that's fine; instant failure is
still a test of out-of-band responses overtaking in-band commands.

For 3., open FIFO for writing.

Drop QMP command x-oob-test.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-6-armbru@redhat.com>
[Error checking tweaked]