backup: fix handling backup filename with spaces

Fixes QubesOS/qubes-issues#1371

qubes-rpc/qubes.Restore

@@ -7,7 +7,7 @@ if [ -f "$args" ] ; then
   echo "Performing restore from backup file $args" >&2
   TARGET="$args"
   echo "Copying $TARGET to STDOUT" >&2
-  /usr/lib/qubes/tar2qfile $TARGET $paths
+  /usr/lib/qubes/tar2qfile "$TARGET" $paths
 else
   echo "Checking if arguments is matching a command" >&2
   COMMAND=`echo $args | cut -d ' ' -f 1`