security/vuxml: Improve newentry

[fernape]

Summary

Rearrange the code a bit by introducing providers (MITRE, NVDE and EUVD).
Fields are retrieved from providers in an orderly fashion.
Should a provider fail to return a value, the next in the list is queried.

While here relax a bit the warnings about the format which are already shown in the make validate target anyway.

Test Plan

Apply the patch and run make newentry CVE_ID=CVE-YYYY-XXXXX for some valid CVE.