Fix HA Peer Serial location

[devyntk]

Motivation and Context

During testing and attempts to create HA pairs during initialization of a device tree from Panorama, I noticed that I was unable to get the HA peer serial, and after some investigation I found that this path was incorrect from the XML I was able to test against.

How Has This Been Tested?

This has been run against an environment with several Panorama and Firewall devices on PAN-OS 10.2.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes if appropriate.
• [ ] All new and existing tests passed.

[FliesLikeABrick]

@shinmog @btorresgil long time no talk - would either of you be available for review?