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).

Start with xenserver/xenserver84/cloud-plugin-storage and inspect the import failure reported by XCP-ng 8.3, then review linked pull request #13692. Reproduce the host-plugin call for mountNfsSecondaryStorage and verify that secondary storage mounts and system VMs deploy without masking storage operations that still require the refactored module.

Written by the indexing model from the issue text.