add xvfb config and startup

Partially fixes #212 - the other part is in https://github.com/xtuple/xtuple-server-core/pull/492
This PR can't pass CI until https://github.com/xtuple/xtuple-server-core/pull/492 is merged.
btw, `assert(procs.length >= 1)` is equivalent here, since `(N >=1) ==> false` for `N ∈ { 0, false, null, undefined }`