1. if the VM is created from instanceType, edit rootdisk, it cannot uncheck "Use this disk as a boot source". And if other disk is bootable, check "Use this disk as a boot source" on rootdisk is also not possible.
2. if the VM is created from template, edit rootdisk and uncheck "Use this disk as a boot source", the "bootable" flag is still on rootdisk.

4.16

1. create vm from instancetype
2. try to uncheck "Use this disk as a boot source" on rootdisk, cannot save
3. edit cloud-init disk, check "Use this disk as a boot source" and save is okay
4. go back to rootdisk, check "Use this disk as a boot source" is not able to save