iotests/graph-changes-while-io: New test

iotests/graph-changes-while-io: New test

Test the following scenario:
1. Some block node (null-co) attached to a user (here: NBD server) that
   performs I/O and keeps the node in an I/O thread
2. Repeatedly run blockdev-add/blockdev-del to add/remove an overlay
   to/from that node

Each blockdev-add triggers bdrv_refresh_limits(), and because
blockdev-add runs in the main thread, it does not stop the I/O requests.
I/O can thus happen while the limits are refreshed, and when such a
request sees a temporarily invalid block limit (e.g. alignment is 0),
this may easily crash qemu (or the storage daemon in this case).

The block layer needs to ensure that I/O requests to a node are paused
while that node's BlockLimits are refreshed.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20220216105355.30729-4-hreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>