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##### SUMMARY
<!--- Describe the new feature/improvement briefly below -->
vmware_guest_disk should propose `disk_mode` parameter (like vmware_guest module).
The playbook bellow does not raise an error but disk is always added in `persistent` mode, even I set `independent_persistent` for `disk_mode` var.
If it's a bug please could you fix it? If it's not consider it as a feature request :)

##### ISSUE TYPE
- Feature Idea (or maybe a bug)

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
vmware_guest_disk 

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml
---
- name: get vm {{ target | upper }} facts
  vmware_guest_facts:
    hostname: "{{ vcenter_hostname }}"
    username: "{{ vcenter_username }}"
    password: "{{ vcenter_password }}"
    datacenter: "{{ datacenter }}"
    validate_certs: no
    name: "{{ target | upper }}"
  register: vm_facts
  delegate_to: localhost

- name: add disk to vm {{ target | upper }}
  vmware_guest_disk:
    hostname: "{{ vcenter_hostname }}"
    username: "{{ vcenter_username }}"
    password: "{{ vcenter_password }}"
    datacenter: "{{ datacenter }}"
    validate_certs: no
    name: "{{ target | upper }}"
    disk:
      - size_gb: "{{ size_gb }}"
        type: "{{ type }}"
        disk_mode: "{{ disk_mode }}"
        scsi_controller: "{{ scsi_controller }}" 
        unit_number: "{{ unit_number }}"
        datastore: "{{ vm_facts.instance.hw_datastores | first }}"
  delegate_to: localhost
```

<!--- HINT: You can also paste gist.github.com links for larger files -->

"""
        Gather facts about VM's disks
        Args:
            vm_obj: Managed object of virtual machine
        Returns: A list of dict containing disks information
        """
        disks_facts = dict()
        if vm_obj is None:
            return disks_facts
        disk_index = 0
        for disk in vm_obj.config.hardware.device:
            if isinstance(disk, vim.vm.device.VirtualDisk):
                disks_facts[disk_index] = dict(
                    key=disk.key,
                    label=disk.deviceInfo.label,
                    summary=disk.deviceInfo.summary,
                    backing_filename=disk.backing.fileName,
                    backing_datastore=disk.backing.datastore.name,
                    backing_disk_mode=disk.backing.diskMode,
                    backing_writethrough=disk.backing.writeThrough,
                    backing_thinprovisioned=disk.backing.thinProvisioned,
                    backing_eagerlyscrub=bool(disk.backing.eagerlyScrub),
                    controller_key=disk.controllerKey,
                    unit_number=disk.unitNumber,
                    capacity_in_kb=disk.capacityInKB,
                    capacity_in_bytes=disk.capacityInBytes,
                )
                disk_index += 1
        return disks_facts
def main():