virtio-scsi-test: Test attaching new overlay with iothreads

This tests that blockdev-add can correctly add a qcow2 overlay to an
image used by a virtio-scsi disk in an iothread. The interesting point
here is whether the newly added node gets correctly moved into the
iothread AioContext.

If it isn't, we get an assertion failure in virtio-scsi while processing
the next request:

    virtio_scsi_ctx_check: Assertion `blk_get_aio_context(d->conf.blk) == s->ctx' failed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>