Detaches disk from the VM.
If the persistent disk is attached to a VM that will be deleted, it's more likely
delete_vm CPI method will be called without a call to
detach_disk with an expectation that
delete_vm will make sure disks are disassociated from the VM upon its deletion.
Agent settings should have been updated to remove information about given disk.
vm_cid[String]: Cloud ID of the VM.
disk_cid[String]: Cloud ID of the disk.
No return value