Refactored X server discovery during provisioning

xset q wasn't working, so go back to hard-coded process name search, but with polling to deal with systemd delaying the launch of the X server process.

requested review from @vohilshe

mentioned in commit 6b3df103

merged

Somehow the fallback logic now doesn't kick in when bringing up a linux machine on AWS; X fails to start (or perhaps rather doesn't keep running) because there's no display, which is the main reason for the entire Xvfb shenanigans. So the process might go away again quickly, for which I needed to add a bit more tinkering on top of your patch. But now it seems to work in all situations.