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>
2 files changed