proxmox_kvm: allow setting format to null (None) (#1028)

* Allow setting format to null (None)

* Use string instead of None for consistency

* Add changelog

* Update changelogs/fragments/1028-proxmox-kvm-linked-clone.yml

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/modules/cloud/misc/proxmox_kvm.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Substitute 'unspecified' format with None

Co-authored-by: Lukasz Rzasik <lukasz.rzasik@dreamlab.net>
Co-authored-by: Felix Fontein <felix@fontein.de>

changelogs/fragments/1028-proxmox-kvm-linked-clone.yml

@@ -0,0 +1,3 @@
+---
+bugfixes:
+  - proxmox_kvm - fix issue causing linked clones not being create by allowing ``format=unspecified`` (https://github.com/ansible-collections/community.general/issues/1027).

plugins/modules/cloud/misc/proxmox_kvm.py

@@ -120,9 +120,10 @@ options:
     description:
       - Target drive's backing file's data format.
       - Used only with clone
+      - Use I(format=unspecified) and I(full=false) for a linked clone.
     type: str
     default: qcow2
-    choices: [ "cloop", "cow", "qcow", "qcow2", "qed", "raw", "vmdk" ]
+    choices: [ "cloop", "cow", "qcow", "qcow2", "qed", "raw", "vmdk", "unspecified" ]
   freeze:
     description:
       - Specify if PVE should freeze CPU at startup (use 'c' monitor command to start execution).
@@ -472,6 +473,22 @@ EXAMPLES = '''
     format: qcow2
     timeout: 500
 
+- name: >
+    Create linked clone VM with only source VM name.
+    The VM source is spynal.
+    The target VM name is zavala
+  community.general.proxmox_kvm:
+    api_user: root@pam
+    api_password: secret
+    api_host: helldorado
+    clone: spynal
+    name: zavala
+    node: sabrewulf
+    storage: VMs
+    full: no
+    format: unspecified
+    timeout: 500
+
 - name: Clone VM with source vmid and target newid and raw format
   community.general.proxmox_kvm:
     api_user: root@pam
@@ -865,7 +882,7 @@ def main():
             description=dict(type='str'),
             digest=dict(type='str'),
             force=dict(type='bool', default=False),
-            format=dict(type='str', default='qcow2', choices=['cloop', 'cow', 'qcow', 'qcow2', 'qed', 'raw', 'vmdk']),
+            format=dict(type='str', default='qcow2', choices=['cloop', 'cow', 'qcow', 'qcow2', 'qed', 'raw', 'vmdk', 'unspecified']),
             freeze=dict(type='bool'),
             full=dict(type='bool', default=True),
             hostpci=dict(type='dict'),
@@ -945,6 +962,9 @@ def main():
     vmid = module.params['vmid']
     validate_certs = module.params['validate_certs']
 
+    if module.params['format'] == 'unspecified':
+        module.params['format'] = None
+
     # If password not set get it from PROXMOX_PASSWORD env
     if not api_password:
         try: