iotests: add test cases for failing flush after qemu-img convert In preparation to add more checks in the following commits and show how the behavior changes. Signed-off-by: Fiona Ebner <f.ebner@proxmox.com> Message-ID: <20260930132140.2342574-2-f.ebner@proxmox.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Eric Blake <eblake@redhat.com>