Update iosxr.rb with admin keyword for show inventory command

[maugli13]

Pre-Request Checklist

• [] Passes rubocop code analysis (try rubocop --auto-correct)
  The core code didn't change. Only specific model
• Tests added or adapted (try rake test)
  The core code didn't change. Only specific model
• Changes are reflected in the documentation
  Particular Model didn't have the documentation
• User-visible changes appended to CHANGELOG.md
  N/A

Description

"show inventory" command without admin keyword is not showing some useful inventory information, such as FAN, PSU and Chassis serial and part numbers

[aschaber1]

Does it show nothing or just less information?
Does admin need elevated privileges?

[maugli13]

@aschaber1 without admin command "show inventory" providing less information.
For example, FAN, PSUs, and Chassis related data are absent.

Yes, admin mode requires root-system group access privileges; commands executed by the elevated users can be limited to RO by AAA or any other options.

[robertcheramy]

As "show running-config" already needs elevated privileges, I think "admin show inventory" should be OK.