iotests: fix 240

iotests: fix 240

Commit 2e8e18c2e463 ("virtio-scsi: add iothread-vq-mapping parameter")
removed the limitation that virtio-scsi devices must successfully set
the AioContext on their BlockBackends. This was made possible thanks to
the QEMU multi-queue block layer.

This change broke qemu-iotests 240, which checks that adding a
virtio-scsi device with a drive that is already in another AioContext
will fail.

Update the test to take the relaxed behavior into account. I considered
removing this test case entirely, but the code coverage still seems
valuable.

Fixes: 2e8e18c2e463 ("virtio-scsi: add iothread-vq-mapping parameter")
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Message-ID: <20250529203147.180338-1-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 2e887187454e57d04522099d4f04d17137d6e05c)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>