feat(hack): add scripts to drive the dev environment headless

Start QEMU in the background with a QMP socket and console logs, take
screenshots, press keys and wait for serial output, so the graphical boot
menu can be driven without a terminal or a window. boot.sh gets opt-in
knobs for this and behaves as before otherwise.

hack/dev/run/stop.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+# Stops QEMU and removes its socket. Safe to run when nothing is running.
+set -e
+
+cd "$(dirname "${0}")/../../.." || exit 1
+
+. "hack/dev/run/lib.sh"
+
+if is_running; then
+	PID="$(cat "${PID_FILE}")"
+	./hack/dev/run/qmp.sh '{"execute":"quit"}' >/dev/null 2>&1 || true
+	for _ in 1 2 3 4 5 6 7 8 9 10; do
+		is_running || break
+		sleep 0.5
+	done
+	is_running && kill "${PID}" 2>/dev/null || true
+fi
+
+rm -f "${QMP_SOCK}" "${PID_FILE}"