Fix Static NAT/Port Forwarding when VM NIC is not the default

[sudo87]

Description

This PR intends to fix #8366

When CloudStack VR forwards traffic to a VM whose NIC is not the VM's default, the VM may reply via its default NIC (asymmetric routing). To force symmetric routing we SNAT the forwarded packet's source to the VR’s guest IP on the VM subnet so the VM replies on-link back to that VR interface. Also fix the port-forwarding SNAT to pick the correct VR guest IP for the VM's subnet

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

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

1. Deploy instance vm1 on network (net_1)
2. Create Isolated network (iso_2), attach NIC nic_2 from this to vm1
3. Acquire public_ip from iso_2 and enable static nat to vm1
4. Configure firewall rules e.g. enable icmp
5. Ping via public_ip, expect response from instance vm1

How did you try to break this feature and the system with this change?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 3.51%. Comparing base (3c1f031) to head (6d8746f).
⚠️ Report is 3 commits behind head on main.

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HEAD has 1 upload less than BASE

Flag	BASE (3c1f031)	HEAD (6d8746f)
unittests	1	0

Additional details and impacted files

@@              Coverage Diff              @@
##               main   #13200       +/-   ##
=============================================
- Coverage     18.08%    3.51%   -14.57%     
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  Files          6037      464     -5573     
  Lines        542611    40154   -502457     
  Branches      66433     7559    -58874     
=============================================
- Hits          98136     1413    -96723     
+ Misses       433448    38551   -394897     
+ Partials      11027      190    -10837     

Flag	Coverage Δ
uitests	3.51% <ø> (ø)
unittests	?

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[weizhouapache]

1. this should apply to additional nics of guest VMs only. since there is no issue with first NIC of guest VMs, it is not needed.
2. it would be better to provide an option for end users.

[sudo87]

@blueorangutan package

[blueorangutan]

@sudo87 a [SL] Jenkins job has been kicked to build packages. It will be bundled with no SystemVM templates. I'll keep you posted as I make progress.

[blueorangutan]

Packaging result [SF]: ✔️ el8 ✔️ el9 ✔️ el10 ✔️ debian ✔️ suse15. SL-JID 17959

[sudo87]

@blueorangutan package

[blueorangutan]

@sudo87 a [SL] Jenkins job has been kicked to build packages. It will be bundled with no SystemVM templates. I'll keep you posted as I make progress.