systemvm: enable IPv6 link-local on the control network

[wido]

Description

This PR enables IPv6 link-local addressing on the control (link-local) network of system VMs, alongside the existing 169.254.0.0/16 IPv4 addressing. With IPv6 explicitly enabled on the control network we can move our SSH commands (on at least KVM) to link local IPv6 and remove the 169.254.0.0/16 addressing in the future. That can significantly reduce the code-base inside CloudStack. That's not part of this PR yet.

The existing issue #9957 talks about this and this PR should be groundwork for that to be done in the future.

System VM (common.sh):

• The control NIC gets an IPv6 link-local address generated with EUI-64 (addr_gen_mode=0), so the address is deterministic and can be calculated from the NIC's MAC address
• Router Advertisements and SLAAC are disabled on the interface (accept_ra=0, autoconf=0); only the link-local address is configured
• setup_sshd() waits for Duplicate Address Detection to complete and adds a ListenAddress fe80::...%ethX entry to sshd_config, so sshd listens on both the IPv4 and IPv6 link-local addresses. The sed expressions are idempotent across reboots
• This works for all system VM types, as setup_sshd() is always handed the control interface (eth0 for CPVM/SSVM on KVM, eth1 for routers)

Management Server / API:

• listSystemVms now returns linklocalip6: the EUI-64 IPv6 link-local address calculated from the control NIC's MAC address using the existing (unit-tested) NetUtils.ipv6LinkLocal(). This performs the same U/L-bit flip + ff:fe expansion the kernel does, so the reported address always matches what the system VM configures
• New response field on SystemVmResponse (since 4.23.0)

KVM agent:

• When the control network is set up, the agent now enables IPv6 (link-local only, no RA/SLAAC) on the control bridge (cloud0) via a shared enableBridgeIpv6LinkLocal() helper in VifDriverBase, called from BridgeVifDriver, OvsVifDriver and IvsVifDriver
• This runs unconditionally on agent start, so hosts with a pre-existing cloud0 bridge also get IPv6 enabled on it
• With this, the host can reach system VMs over the control network with e.g. ssh -6 -p 3922 fe80::...%cloud0

UI:

• The System VMs list and detail views show the new "Link-local/Control IPv6 address" field

This is a first step towards using IPv6 link-local instead of 169.254.0.0/16 on the control network. The Management Server still provisions system VMs over IPv4; nothing changes for existing deployments.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• build/CI
• test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

How Has This Been Tested?

• api, server and plugins/hypervisors/kvm compile cleanly
• bash -n on common.sh
• NetUtils.ipv6LinkLocal() is covered by existing unit tests in NetUtilsTest (testIpv6LinkLocal), verifying the EUI-64 calculation matches the kernel's

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.65%. Comparing base (4f11707) to head (137b3da).

Files with missing lines	Patch %	Lines
...ache/cloudstack/api/response/SystemVmResponse.java	0.00%	6 Missing ⚠️
...m/cloud/hypervisor/kvm/resource/VifDriverBase.java	0.00%	4 Missing ⚠️
...src/main/java/com/cloud/api/ApiResponseHelper.java	0.00%	2 Missing ⚠️
...cloud/hypervisor/kvm/resource/BridgeVifDriver.java	0.00%	1 Missing ⚠️
...om/cloud/hypervisor/kvm/resource/IvsVifDriver.java	0.00%	1 Missing ⚠️
...om/cloud/hypervisor/kvm/resource/OvsVifDriver.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main   #13795   +/-   ##
=========================================
  Coverage     19.65%   19.65%           
  Complexity    19792    19792           
=========================================
  Files          6368     6368           
  Lines        574881   574896   +15     
  Branches      70351    70352    +1     
=========================================
+ Hits         112970   112975    +5     
- Misses       449639   449648    +9     
- Partials      12272    12273    +1     

Flag	Coverage Δ
uitests	3.41% <ø> (ø)
unittests	20.92% <0.00%> (+<0.01%)	⬆️

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