Document another workaround for a vagrant failure

Originally created by @segfault on #16144 (Redmine)

When I try to build Tails, vagrant sometimes fails with

Initialization parameters must be an attributes hash, got NilClass nil (ArgumentError)

Restarting libvirtd fixes this, as suggested here.

We should add this to our Known issues and workarounds section in contribute/build.

Feature Branch: bugfix/16144-document-workaround