fix: Add comment (#1368)

src/power.sh

@@ -21,6 +21,7 @@ finish() {
 
   local reason=$1 failed=0
 
+  # A nonzero exit is unexpected only when QEMU_END is missing.
   if [ ! -f "$QEMU_END" ] && (( reason != 0 )); then
     failed=1
   fi