[FIX] base_vat: properly checks XI VAT numbers

In 12.0+ when adding an XI (Northern Ireland) VAT number on a vendor and specifying the country as United Kingdom, an error shows the VAT number as not valid. This is because the method to check specifically XI VAT numbers doesn't recognize XI as a country, and thus uses the GB VAT number verification, which doesn't recognize XI VAT numbers as it isn't up to date yet.

A temporary method to check XI VAT number was already added, but is never called because XI is not recognized as a country code.
With this commit, we add a list of known legitimate country codes, that are not considered as such in Odoo.

opw-2534541

closes odoo/odoo#72910

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>