prepare-dvm: fix bashism

$(( )) is POSIX syntax for shell arithmetic operations. Especially dash
(default shell in Debian) doesn't support $[ ].

vm-systemd/prepare-dvm.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 possibly_run_save_script()
 {
@@ -17,7 +17,7 @@ if true; then
     qubesdb-watch /qubes-restore-complete &
     watch_pid=$!
     free | grep Mem: | 
-        (read label total used free shared buffers cached; qubesdb-write /qubes-used-mem $[ $used + $cached ])
+        (read label total used free shared buffers cached; qubesdb-write /qubes-used-mem $(( $used + $cached )) )
     # we're still running in DispVM template
     echo "Waiting for save/restore..."
     qubesdb-read /qubes-restore-complete || wait $watch_pid