T7072: VPP CPU skip-cores should be verified

[natali-rs1985]

Change Summary

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)
• Other (please describe):

Related Task(s)

• https://vyos.dev/T7072

Related PR(s)

Proposed changes

How to test

set vpp settings interface eth2 driver dpdk
set vpp settings cpu skip-cores 99

vyos@vyos# commit
[ vpp ]
The system does not have enough available CPUs to skip (reduce "cpu
skip-cores" to 3 or less)
[[vpp]] failed
Commit failed
[edit]
vyos@vyos# set vpp settings cpu skip-cores 2
[edit]
vyos@vyos# commit
[edit]
vyos@vyos#

Checklist:

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