Sflow: T7175: Added VPP option

[ServerForge]

Change summary

Updated hsflowd CLI to have VPP option and updated error check to handle interface checks when using VPP.

vyos@vyos# set system sflow 
Possible completions:
   agent-address        sFlow agent IPv4 or IPv6 address
   agent-interface      IP address associated with this interface
   drop-monitor-limit   Export headers of dropped by kernel packets
   enable-egress        Enable egress sampling
   enable-vpp           Enable VPP sampling
+  interface            Interface
   polling              Schedule counter-polling in seconds (default: 30)
   sampling-rate        sFlow sampling-rate (default: 1000)
+> server               sFlow destination server
   vrf                  VRF instance name

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T7175

Related PR(s)

vyos/vyos-build#962
vyos/vyos-vpp#32

How to test / Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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CI integration 👍 passed!

Details

CI logs

• CLI Smoketests (no interfaces) 👍 passed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests 👍 passed
• RAID1 tests 👍 passed
• TPM tests 👍 passed