XenServer / XCP-ng hypervisor resource, Storage (secondary storage mount / system VM deployment)

### problem

On a fresh deployment with XCP-ng 8.3 hosts, the secondary storage VM and console proxy VM never deploy. They fail and retry in a loop with ascending VM ids. The management log reports `Could not mount secondary storage :/`, which is misleading: NFS connectivity, exports, and permissions are all fine (the share mounts manually from both the hosts and the management server without issue).

The real cause is that the host-side XAPI plugin `/etc/xapi.d/plugins/cloud-plugin-storage` fails to load on XCP-ng 8.3 because it does a top-level `import lvhdutil`, and `lvhdutil` no longer exists as an importable module on XCP-ng 8.3. Because the plugin crashes at import time, every storage operation that goes through it fails, which CloudStack surfaces as a mount failure and a "storage unreachable" condition.

The plugin deployed to XCP-ng 8.3 hosts is sourced from the `xenserver84` directory:
`.../xenserver/xcpserver83/../xenserver84/cloud-plugin-storage`.

### versions

~~~
4.22.0.0
Fully patched XCP-ng 8.3 servers
~~~

### The steps to reproduce the bug

System VMs cycle endlessly with ascending ids. The management log shows, per attempt:

~~~
WARN [c.c.h.x.r.XcpServer83Resource] callHostPlugin failed for cmd: mountNfsSecondaryStorage
with args remoteDir: :/, localDir: /var/cloud_mount/, nfsVersion: null,
due to Task failed!
WARN [c.c.h.x.r.Xenserver625StorageProcessor] ... Could not mount secondary storage :/ ...
ERROR [o.a.c.e.o.VolumeOrchestrator] Unable to create volume [ROOT-xxx] ...
WARN [c.c.v.ClusteredVirtualMachineManagerImpl] Resource [StoragePool:x] is unreachable ...
~~~

The underlying XAPI task error reveals the actual failure (host-side Python traceback):

~~~
errorInfo: [XENAPI_PLUGIN_FAILURE, non-zero exit, , Traceback (most recent call last):
File "/etc/xapi.d/plugins/cloud-plugin-storage", line 34, in
import lvhdutil
ModuleNotFoundError: No module named 'lvhdutil'
]
~~~

On the XCP-ng 8.3 host, the `lvhdutil` source module is absent — only a stale orphan bytecode file remains, which Python 3 will not import:

~~~
# find /opt/xensource/sm /usr/lib/python* -name 'lvhdutil*'
/opt/xensource/sm/__pycache__/lvhdutil.cpython-36.pyc

# ls /opt/xensource/sm/ | grep -i lvhd
(no source .py present)
~~~

Note the other imports just above `import lvhdutil` (`SR`, `VDI`, `SRCommand`, `util`, `lvutil`, `vhdutil`) succeed, which confirms `/opt/xensource/sm` is on the path and that `lvhdutil` specifically has been removed/refactored out in XCP-ng 8.3.

### What to do about it?

Make the import non-fatal in
`/usr/share/cloudstack-common/scripts/vm/hypervisor/xenserver/xenserver84/cloud-plugin-storage`
on the management server, then push the patched plugin to the hosts (the management server only copies host plugins on host add, not on reconnect/restart, so a manual push or host re-add is required):

~~~python
import vhdutil
import shutil
try:
import lvhdutil
except ImportError:
lvhdutil = None
import errno
~~~

After this, the NFS mount succeeds and the system VMs deploy normally.

##### OPEN QUESTION / SUGGESTED FIX
The guard above unblocks NFS and basic operation, but it does not restore any plugin code path that genuinely needs `lvhdutil` (relevant to LVM/iSCSI primary storage operations such as snapshots and template-from-volume). The proper fix is likely to import `lvhdutil` from its new location in XCP-ng 8.3's refactored `sm` stack (or call its replacement), rather than guarding it to `None`. Guidance on the correct module/path for 8.3 would be appreciated.

##### RELATED
Possibly related to discussion #11346 (4.20.1 + XCP-ng 8.3 system VMs failing to deploy), though that case was resolved as an NFSv4 negotiation issue on the storage server — this is a distinct root cause (plugin import failure).

xenserver/xenserver84/cloud-plugin-storage から始めて、XCP-ng 8.3 で報告されたインポート失敗を調査し、その後、リンクされた pull request #13692 を確認します。mountNfsSecondaryStorage の host-plugin 呼び出しを再現し、リファクタリングされたモジュールを引き続き必要とするストレージ操作を隠すことなく、セカンダリストレージがマウントされ、システム VM がデプロイされることを検証します。

索引モデルが issue の本文から書いたものです。