stop-target: use poweroff instead of halt

When running Debian Jessie in KVM, using the halt command shuts down
all services but does not stop the qemu process. Using the poweroff
command avoids this problem.

libexec/stop-target

@@ -11,7 +11,7 @@ case $VMSW in
   KVM)
     if [ ! -e var/target.pid ]; then exit; fi
 
-    on-target -u root halt
+    on-target -u root poweroff
     sleep 5
 
     if [ ! -e var/target.pid ]; then exit; fi