iotests: Add iothread cases to 155

iotests: Add iothread cases to 155

This patch adds test cases for attaching the backing chain to a mirror
job target right before finalising the job, where the image is in a
non-mainloop AioContext (i.e. the backing chain needs to be moved to the
AioContext of the mirror target).

This requires switching the test case from virtio-blk to virtio-scsi
because virtio-blk only actually starts using the iothreads when the
guest driver initialises the device (which never happens in a test case
without a guest OS). virtio-scsi always keeps its block nodes in the
AioContext of the the requested iothread without guest interaction.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200310113831.27293-7-kwolf@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>