examples/deploy-qemu-vm: show progress when waiting

This improves the user experience on VM startup, which can take a few
seconds.

examples/qemu-vm/wait-until.sh

@@ -0,0 +1,21 @@
+# Wait until $condition is true, retrying every $intervalMs milliseconds.
+# Print a '.' character every second as a progress indicator.
+waitUntil() {
+    condition=$1
+    intervalMs=$2
+
+    lastDotTime=$(getTimeMs)
+    while ! { t0=$(getTimeMs); eval "$condition"; }; do
+        now=$(getTimeMs)
+        if ((now - lastDotTime >= 1000)); then
+            printf .
+            lastDotTime=$now
+        fi
+        toSleep=$((t0 + intervalMs - now))
+        if ((toSleep > 0)); then
+            sleep $((toSleep / 1000)).$((toSleep % 1000));
+        fi
+    done
+}
+
+getTimeMs() { date +%s%3N; }