shell-completion: use list-images rather than list

"machinectl list" only lists running machines while many of the MACHINES
commands use names of images; both running and non-running.

List machines from both "list" and "list-images" and use sort -u to
avoid duplicates.

shell-completion/bash/machinectl

@@ -26,7 +26,8 @@ __contains_word() {
 
 __get_machines() {
         local a b
-        machinectl list --no-legend --no-pager | { while read a b; do echo " $a"; done; };
+        (machinectl list-images --no-legend --no-pager; machinectl list --no-legend --no-pager) | \
+		{ while read a b; do echo " $a"; done; } | sort -u;
 }
 
 _machinectl() {